When a black female proceeds to laugh, cackle, and screech at the same time- signalling an impending attack. It is often done by younger black women, but older ones may also do this if provoked.
To "brack attack" is a verb, it means to put ones hands down a girls pant and feel both of her ass cheeks in a cupping motion. This maneuver can be performed in a number of different situations. The prime situation in which you would use this is when you are hooking up with a girl.
guy 1: dude last night i was hooking up with this girl who had the nicest ass ever.