Ball your fist up, and lightly "bop" the bottom, or soft part of it, on someone's forehead after saying the words, "Ball up!" it's a verbal and physical response to someone who fails/messes up/is just plain wrong/ or does something stupid when trying to make themselves look cool or smart. Some people even say, "bink!" while simultaneously applying the forehead bop.
"Boise State deserves to play for a National Championship."
"Ball up!"
"I tried to get that chicks number, but she told me she'd rather swallow gasoline than continue the conversation..."
1. a command by which someone tricks another at looking in the direction of the hand in an "OK" sign, by which the perpetrator must clench their fist to allow the fooling party to strike it hard in a slapping motion.