The verbal greeting equivalent to a hand
shake. "Smacking" comes from the sound peoples hands make when they slap hands during a handshake. 'Off' is the from the saying "Whats poppin off" which means "whats going down? or whats up?". Smacking off is the greeting that is a combination of the
two, a handshake and a question
rolled into one saying, used as a greeting amongst friends or associates.