Green’s Dictionary of Slang

(a) (US prison) a prize-fighter.

1904		‘Number 1500’ Life In Sing Sing 250: Jabber. Prizefighter.
1909		‘The Lang. of Crooks’ in Wash. Post 20 June 4/1: [paraphrasing J. Sullivan] A shaming pusher or a jabber is a prize fighter.
1949		Monteleone Criminal Sl. (rev. edn).

(b) any person who is prone to fighting.

1911		Gem 16 Sept. 7: You biffed them over, you burbling jabber!
1999		E. Bunker Mr Blue 28: The attendant in charge at night was named Hunter but he was called Jabber.

(a) (US prison) a hypodermic syringe.

1918

H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 28/2: JABBER. Hypodermic needle.

(b) (US drugs) a drug addict.

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 104/1: jabber. A needle-addict.
1950		Goldin et al. DAUL 109/1: Jabber. A drug addict who takes hypodermic injections; a hypo; a needle-knight.
1959		J.E. Schmidt Narcotics Lingo and Lore.