back five

When someone offers up a high five and you give them the back of your hand instead of the palm. Could also be described as a "half a five."

A team scores and some dude says "high five man!" then as you are giving the high five turn your hand around to give the "back five." Maybe because you want to be cool with that person (not leave them hanging) but you're just not into it - it's like a weak high-five or a half-high-five.