The feeling one gets after participating in a) Challenging test, b) Long, strenuous test, and/or c) A test in which one had no clue about what any of the questions were about. The feeling includes tiredness for a few days, induced feelings of senioritis, the will to say fuck school, headaches, and sluggish-mindedness afterwards for awhile.
Jerry: Dude, what happened to you? You look exhausted!
Lean: I just had an AP test yesterday, I'm on such a testing hangover..
Jerry: So I guess we ain't hanging out today?
Lean: Hell no, I need more sleep, I wish school were to end, and I have a massive headache!