When a security researcher describes a user action as lethal, they typically mean it triggers a condition that irreversibly compromises the system's integrity, confidentiality, or availability—often without recourse for mitigation after the fact. This could include actions like clicking a malicious link that installs a rootkit, reusing credentials on a phishing site leading to credential stuffing, or executing a command that bricks a device. "Lethal" underscores not just the severity but also the finality: the action doesn't just degrade security but catastrophically collapses it, often silently and instantly.