a "handshake" of sorts, used in greeting or in congratulations. The hand is held in a first. In the longer version, one person taps the bottom of their fist against the top of the other person's fist, then the fists are to switch place and are tapped again, then they are tapped front-to-front.
In the shorter version, the closed fists are simply tapped front-to-front.
This front-to-front version became popular in a news clip of Michelle and Barack Obama. Fox News host E.D. Hill asked what to describe the maneuver, suggesting "terrorist fist jab."
I just stood there looking like a moron with my hand out, but he wouldn't look over and give me dap.