Phyg is the rot13 cipher of "cult". At some point in LessWrong's history, there was concern that discussion of cults on the site would cause "LessWrong" to rank highly in Google search results for "cult". It was therefore recommended that people instead use the word, "phyg", to discuss cults.
Cults have been discussed on LessWrong in part because people wondered whether or not LessWrong was maybe a cult – there was enough enthusiasm around how it radically changed your life, and there sure was a charismatic leader.
The rest of this page is the contents of the Phyg page from the old LessWrong wiki:
Less Wrong may or may not be a cult phyg.1 2
Eliezer Yudkowsky is not the leader3
Nor is singularianism.4
Well maybe RationalWiki disagrees