when someone puts an empty case of beer on his (almost always male) head. generally what the drunkest douche at any given party inevitably ends up wearing. extreme case hattees may even poke eye and mouth holes in their case hat, for prolonged case hatting.
Bro, you shoulda been at Rick's party last night. Tim got totally axed and put on a case hat, and then he fell down and we beat the shit out of him with a big wooden spoon.