Green’s Dictionary of Slang

1. (orig. boxing) a conspicuously weak jaw, which breaks or fractures when hit and loses its possessor his fights; thus glass-jawed adj; also in fig. use.

1917		T.A. Dorgan in Zwilling TAD Lex. (1993) 40: Why, listen, those glass jawed champions go just so far. And then back to the hod and the overalls.
1921		H.C. Witwer Leather Pushers 34: The guy with the glass jaw or the weak muscled stomach.
1931		Irwin Amer. Tramp and Und. Sl. 86: Glass Jaw. – [...] a weak or sensitive chin, one upon which a comparatively light blow is enough to cause a ‘knock out,’ no matter how valorous the fighter.
1938		T. Thursday ‘Tomahawk McCloskey’ in Thrilling Sports Mar. 🌐 Six-Sock Smelli [...] has a jaw that was manufactured by one of the leading glass companies.
1940		R. Chandler Farewell, My Lovely (1949) 148: You’re a tough guy. Six feet of iron man [...] Hard muscles and no glass jaw. You can take it.
1947		B. Schulberg Harder They Fall (1971) 47: Looked tough, but he was muscle-bound, had a glass jaw.
1962		Ragen & Finston World’s Toughest Prison 801: glass jaw – A weak or sensitive chin.
1977		(con. 1949) J.G. Dunne True Confessions (1979) 152: Glass hands, that was always my problem [...] Some guys got a glass jaw, I had glass hands. Knuckles like potato chips.
1989		J. Morton Lowspeak.
2000		M.J. Lasky Lang. of Journalism 14: Bill Clinton still has a glass jaw. All we need to do is land the right punch.

2. (US campus) a coward.

1927		‘Jargon of the Und.’ in DN V 448: Glass jaw, One who is cowardly.
1931		Irwin Amer. Tramp and Und. Sl. 86: Glass Jaw. — A coward; one easily defeated.
1949		Monteleone Criminal Sl. (rev. edn).
1962		Ragen & Finston World’s Toughest Prison 801: glass jaw – A coward; one easily defeated.
1997–2000		Da Bomb 🌐 13: Glass jaw: Coward.
2000		Karter & Mezger Complete Idiot’s Guide to Kickboxing 67: Your only hope is to fight a glass jaw; he’s easily knocked out.