Green’s Dictionary of Slang

(drugs) the occasional use of drugs.

1966		J. Mills Panic in Needle Park (1971) 35: He was only ‘chipping,’ using drugs occasionally when they were handy, and had not yet acquired a habit.
1969		R.R. Lingeman Drugs from A to Z (1970) 61: chipping chippying; chippying [...] infrequent, irregular use of narcotics for pleasure and without physical dependency.
1981		D.E. Miller Bk of Jargon 340: chipping: Also chippying. Nonhabitual, occasional narcotics use for pleasure, among nonaddicts.
2001		ONDCP Street Terms 6: Chipping — Using drugs occasionally.