Green’s Dictionary of Slang

1. (US black teen, also knuck up) to prepare to fight, by closing and raising one’s fists.

1968		G. Cuomo Among Thieves 289: They were the kind of guys that got a big charge out of knuckling guys up.
1985		T.R. Houser Central Sl. 33: knuckle up To fight with fists, one-to-one.
1991		Naughty By Nature ‘Guard Your Grill’ 🎵 Guard your grill, knuckle up, put em up, yup!
2000		Ebonics Primer at www.dolemite.com 🌐 knuckle up Definition: to fight, brawl Example: If you don’t back up off me weez gonna knuckle up!
2012		M. Lacher On the Bro’d 246: ‘[If] dude’s gotta throw down then I just gotta knuck up’.

2. see knuckle v.² (2)