Cooper is a verb that refers to discarding a beer that is not completely finished.
Dude, you coopered the shit out of that beer. It's still half-full.
Don't cooper your beers, boys. We're almost out.
Who keeps leaving these coopered beers on the kitchen counter?
If I catch you coopering your beer again, I'm going to go Slimnasty on your sorry ass.
A male who is obsessed with 4chan and Runescape, you will often hear a Cooper say, "You are a troll", or bring up some joke that only he and a few others may understand. Even though a Cooper can be annoying, he is a generally a decent friend.