A super-pleasant activity --- usually performed as a playful farewell gesture --- whereby you smilingly extend one hand at a time to the other person, then firmly bob both his hands up and down once.
The 1-2-3 handshake is most often administered by a mushy-hearted guy to a pretty girl whom he's met for the first time; it's a way of showing that he really enjoyed the "sit together awhile and chat" session that she shared with him, and hopes that they can continue their friendship for the foreseeable future.
by QuacksO March 2, 2023