Green’s Dictionary of Slang

1. (US drugs) to inject a narcotic, esp. slowly.

1958		W. Motley Let No Man Write My Epitaph (1960) 170: Extra Black Johnson, like so many of them, likes to jag off. [...] Again he injects into his arm, jagging-off, getting a second sensation.
1966		B. Brunner Face of Night 197: You’re the one that jagged off with it, not me [HDAS].

2. (US) to masturbate.

1972		Dahlskog Dict. Contemp. and Colloq. Usage 34/1: Jag off, v. Vulgar: To masturbate.
1973		National Lampoon Aug. 32: If you jag off...hair will sprout on the palm of the hand you jag off with [HDAS].

3. (US) to fool around, to mess about.

1976

N. Ross Policeman’s Bible 63: ‘C’mon, quit jagging off!’ I said; I was irritated [HDAS].

4. (US) to tease or deceive someone.

1974

R. Blount About Three Bricks Shy of a Load 18: Are you just jagging me off, or do you mean that?