Green’s Dictionary of Slang

(US black) to greet someone with a palm-/hand-slapping ritual, where the hands are held low.

1992		Strawberry & Rust Darryl 213: Backslapping, high-fiving, low-fiving, giddy from the release of tension [HDAS].
2002		D.H. Sterry Chicken (2003) 179: I low-five Horse, kiss Cruella, and case the joint.
2016		T. Robinson Rough Trade [ebook] G.G. low-fived Twitch.