Green’s Dictionary of Slang

(drugs) an oily deposit left in a pipe after smoking crack cocaine.

1992		T. Williams Crackhouse 25: When you push the screens down on end of the pipe, you can smoke the ‘res’ down there, because the oils go down on that end.
2001		ONDCP Street Terms 18: Res — Potent residue left as a result of smoking crack which is scraped and smoked.