Epistemic Status: Reference post. Strong beliefs strongly held after much thought, but hard to explain well. Intentionally abstract.
Disambiguation: This does not refer to any physical good, app or piece of software.
Further Research (book, recommended but not at all required, take seriously but not literally): The Book of the Subgenius
Related (from sam[ ]zdat, recommended but not required, take seriously and also literally, entire very long series also recommended): The Uruk Machine
Further Reading (book): Scarcity: Why Having Too Little Means So Much
Previously here (not required): Play in Hard Mode, Play in Easy Mode, Out to Get You
Leads to (I’ve been scooped! Somewhat…): Sabbath Hard and Go Home
An illustrative little game: Carpe Diem: The Problem of Scarcity and Abundance
Slack is hard to precisely define, but I think this comes close:
Definition: Slack. The absence of binding constraints on behavior.
Poor is the person without Slack. Lack of Slack compounds and traps.
Slack means margin for error. You can relax.
Slack allows pursuing opportunities. You can explore. You can trade.
Slack prevents desperation. You can avoid bad trades and wait for better spots. You can be efficient.
Slack permits planning for the long term. You can invest.
Slack enables doing things for your own amusement. You can play games. You can have fun.
Slack enables doing the right thing. Stand by your friends. Reward the worthy. Punish the wicked. You can have a code.
Slack presents things as they are without concern for how things look or what others think. You can be honest.
You can do some of these things, and choose not to do others. Because you don’t have to.
Only with slack can one be a righteous dude.
Slack is life.
Related Slackness
Slack in project management is the time a task can be delayed without causing a delay to either subsequent tasks or project completion time. The amount of time before a constraint binds.
Slack the app was likely named in reference to a promise of Slack in the project sense.
Slacks as trousers are pants that are actual pants, but do not bind or constrain.
Slackness refers to vulgarity in West Indian culture, behavior and music. It also refers to a subgenre of dancehall music with straightforward sexual lyrics. Again, slackness refers to the absence of a binding constraint. In this case, common decency or politeness.
A slacker is one who has a lazy work ethic or otherwise does not exert maximum effort. They slack off. They refuse to be bound by what others view as hard constraints.
Out to Get You and the Attack on Slack
Many things in this world are Out to Get You. Often they are Out to Get You for a lot, usually but not always your time, attention and money.
If you Get Got for compact amounts too often, it will add up and the constraints will bind.
If you Get Got even once for a non-compact amount, the cost expands until the you have no Slack left. The constraints bind you.
You might spend every spare minute and/or dollar on politics, advocacy or charity. You might think of every dollar as a fraction of a third-world life saved. Racing to find a cure for your daughter’s cancer, you already work around the clock. You could have an all-consuming job or be a soldier marching off to war. It could be a quest for revenge, for glory, for love. Or you might spend every spare minute mindlessly checking Facebook or obsessed with your fantasy football league.
You cannot relax. Your life is not your own.
It might even be the right choice! Especially for brief periods. When about to be run over by a truck or evicted from your house, Slack is a luxury you cannot afford. Extraordinary times call for extraordinary effort.
Most times are ordinary. Make an ordinary effort.
You Can Afford It
No, you can’t. This is the most famous attack on Slack. Few words make me angrier.
The person who says “You Can Afford It” is saying to ignore constraints that do not bind you. If you do, all constraints soon bind you.
Those who do not value Slack soon lose it. Slack matters. Fight to keep yours!
Ask not whether you can afford it. Ask if it is Worth It.
Unless you can’t afford it. Affordability is invaluable negative selection. Never positive selection.
The You Can Afford It tax on Slack quickly approaches 100% if unchecked.
If those with extra resources are asked to share the whole surplus, all are poor or hide their wealth. Wealth is a burden and makes you a target. Those visibly flush rush to spend their bounty.
Where those with free time are given extra work, all are busy or look busy. Those with copious free time seek out relatively painless time sinks they can point to.
When looking happy means you deal with everything unpleasant, no one looks happy for long.
The Slackless Like of Maya Millennial
Things are bad enough when those with Slack are expected to sacrifice for others. Things are much worse when the presence of Slack is viewed as a defection.
An example of this effect is Maya Millennial (of The Premium Mediocre Life of Maya Millennial). She has no Slack.
Constraints bind her every action. Her job in life is putting up a front of the person she wants to show people that she wants to be. If her constraints noticeably failed to bind the illusion would fail.
Every action is being watched. If no one is around to watch her, the job falls to her. She must post all to Facebook, to Snapchat, to Instagram. Each action and choice signals who she is and her loyalty to the system. Not doing that this time could mean missing her one chance to make it big.
Maya never has free time. There is signaling to do! At a minimum, she must spend such time on alert and on her phone lest she miss something.
Maya never has spare cash. All must be spent to advance and fit her profile.
Maya lacks free speech, free association, free taste and free thought. All must serve.
Maya is in a world where she must signal she has no Slack. Slack means insufficient dedication and loyalty. Slack cannot be trusted. Slack now means slack later, which means failure. Future failure means no opportunity.
This is more common than one might think.
“Give Me Slack or Kill Me” – J.R. “Bob” Dobbs
The aim of this post was to introduce Slack and give an intuitive picture of its importance.
The short-term practical takeaways are:
Make sure that under normal conditions you have Slack. Value it. Guard it. Spend it only when Worth It. If you lose it, fight to get it back. This provides motivation for fighting things Out To Get You, lest you let them eat your Slack.
Make sure to run a diagnostic test every so often to make sure you’re not running dangerously low, and to engineer your situation to force yourself to have Slack. I recommend Sabbath Hard and Go Home with my take to follow soon.
Also respect the Slack of others. Help them value and guard it. Do not spend it lightly.
A Final Note
I kept this short rather than add detailed justifications. Hopefully the logic is intuitive and builds on what came before. I hope to expand on the details and models later. For a very good book-length explanation of why lacking Slack is awful, see Scarcity: Why Having Too Little Means So Much.

I think a nice, short definition of Slack would be, "A small gradient of the cost function in the vicinity of the optimum." In other words, the penalty for making sub-optimal decisions is not very high, because the region of acceptable behaviors around the best one is fairly large.
And yet here we are, where the penalties for sub-optimal decisions are indeed quite high. The obvious question is, "How did it get to be this way?" Well, typically non-smooth cost functions indicate more complexity, so in some sense the decision landscape has gotten more complicated than it used to be. Intuitively, this would seem to be because there is some sort of adversarial process going on where everyone's strategies have adapted to become more effective.
The narrative that I usually tell myself to explain this goes something like, at some point along the way, firms realized (especially firms that accomplish cognitively demanding tasks) that the majority of their value was being created by a small number of extremely talented people. But these people were not just talented, but also mono-maniacal in their dedication to their jobs. They essentially filled most of their waking hours doing one thing, because they genuinely love it (and they love being hyper-focused). My assumption is that the ratio of the value these people created for their firms over the cost of hiring them was much greater than one. So firms realized that they could cut down on costs heavily by reducing overall hiring and focusing on locating just these people. Therefore they start to look for the signs of these people: Heavy activity outside of work dedicated to their field, a genuine love of it (maybe shown by writing articles, blogs or textbooks, giving talks, etc.), and willingness to work long hours. People adapt by trying to signal these things too even if it means cutting into the time they spend doing things they actually enjoy, but the job market is much tighter now because firms have reduced hiring. In short, firms are winning, people are losing.
But it still leaves the question of why the hyper-talented people are being paid less than they're worth? That seems to be the crux of it all, some kind of market inefficiency that allows firms to win big by getting super-people at a bargain. My naive guess is that it has something to do with the employees themselves, by the nature of their personalities they are actually willing to work for less than they are worth, but I am pretty uncertain about that. I would appreciate any answers from someone with more economic expertise if they have some ideas.
Or if you’re paid by results not the hour, as a contractor, you can earn the same in less time. Or even as an employee, you can just be paid to waste most of your time (though this is fairly unsatisfactory). Eg a friend of mine worked with an excellent programmer who would do nothing for months - literally spend most of the time in the pub or messing around with things that interested him - and occasionally spend a weekend programming furiously to produce what was presented to the (crappy) management as what the entire team had been working on for months.