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.
Person 1 : Smacking off G? What you been up to lately
The act of one giving themselves an over-the-pants handie, whereas they are either unable or too lazy to pull their pants down and masturbate correctly.