Someone who actively decides to do less than their best because either they don't care or they feel the task doesn't deserve the time it would take to do a good job.
"Joe, are you going to put much time into your progress report?"
"Naw, I'm gonna half-ass it, cause nobody will EVER read it anyways".
The act of viewing someone else's half-assmanship and finding oneself so moved to comment upon the splendidness of the half-assmanship that one must utter an exclamation.