Also found in: Acronyms.
slang A paranoid or delusional mindset centered around a belief in some conspiracy theory or another. Refers to the cliché of using such headgear to prevent mind-control or having one's thoughts read. A: "You know that they put those GPS trackers in your phones so the government can monitor you, right?" B: "Dude, you need to take your tinfoil hat off and spend some time away from the Internet for a while." A lot people wearing tinfoil hats believe the moon landing was faked by the US government.
See also: hat