Similar to the idiom, "Wear Your Heart On Your Sleeve" but instead of expressing your emotions freely, you (or someone else) show how much of an asshole you are.
Cal: Yeah, so I punched him in the face for taking the last slice of pizza.
Rudi: It was his birthday, assjacket.
Engaging in stupid activities or stupid behavior. Generally applied to an individual in a light-hearted manner. When a person is being annoying, or making an ass out of himself.
Getting drunk and playing mail-box-baseball is widely considered asscockery.