A cue-card size piece of paper or thin cardboard with the word "ASS" written on it. When one person acts like an ass to another, the ass-ee can give the card to the ass-er as a means of letting the person know that they crossed a line and are just acting like an ass. Generally seen as a peaceful alternative to punching someone in the arm for being an ass. This comes in handy in places where punching is considered inappropriate, such as the workplace, dinner with the in-laws, and church.
A: Hey dude, your mom's cute.
B: Dude, that's weird.
A: Seriously man, she's so hot I'd TOTALLY do her.
B: *Hands ass card over* You crossed a line.