Green’s Dictionary of Slang

1. to be under the influence of drugs.

1959

L. Lipton Holy Barbarians 186: And he hits up the whole thing and he ... starts nodding, you know, hanging. And this other guy, ... he’s hanging.

2. to be in need of some drugs.

1966		B. Brunner Face of Night 232: Hanging—in distress for lack of narcotics. Also, ‘hung up’ [HDAS].
1997		L. Davies Candy 150: We were hanging like dogs, seriously edgy and sweaty.