Knock Bitches Out.
Most likely used in game situations for pump-up purposes.
Girl 1: Let's win this game!
Girl 2: KBO?
Girl 1: Hell yeahhh.
Winston Churchill (British Prime Minister during WWII) ended almost every phonecall with KBO.
KBO is an acronym for "Keep Buggering On"
Winston Churchill: Let's have a drink or two some time after this terrible war is over.
Franklin D. Roosevelt: That would be great fun!
Winston Churchill: Right then, KBO.