Green’s Dictionary of Slang

(drugs) one pound in weight, occas. of flesh.

1969		R.R. Lingeman Drugs from A to Z (1970) 140: L.B. A pound.
1986		Eble Campus Sl. Nov. 6: LB’s – pounds: Susan really put on some LB’s this semester.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 17: I’m pickin’ up an LB later today.
2005		G. Pelecanos Drama City 113: Had it tailored [...] to accommodate these extra el-bees I been carryin’.