glass jaw

An allusion to glass being easily breakable.

glass jaw (plural glass jaws)

1. (chiefly boxing) A fighting vulnerability where one is easily knocked out via a single hard blow to the chin or jaw (due to lack of conditioning, insufficient training, or damage from past cerebral concussions).
   • 1989, Bryce Courtenay, The Power of One, →ISBN, page 109:

     "How was I to know that big ape had a glass jaw?"

   • [2010, Stephen Timblin, Muhammad Ali: King of the Ring, Sterling, →ISBN, page 101:

     Early in Ali's career, experts believed he had what's called a glass jaw—that he would fall after the first big punch. He was now proving that he could take a punch, but each punch chipped away at his health.]

2. (figurative) A vulnerability of that sort; a weak spot.

   Synonyms: see Thesaurus:weak spot

   • 1979, M. LeVan Rhame, Jon Chalmers Niemeyer, Tennis Magic: Playing With a Full Deck, New York: Vantage Press, page 164:

     It is essential that a player be able to identify his glass jaw or his weak spot and learn to deal with it in actual competition.

   • 2008 February 13, Ross Douthat, “Obama's Glass Jaw?”, in The Atlantic‎:

     But if Obama does have a glass jaw, if his candidacy is a bubble waiting to be pricked, a strong Republican nominee like McCain is precisely the guy to do it.

   • 2019 July 1, Gabriela Resto-Montero, “Post-debate polls show Biden’s lead shrinking and Harris gaining”, in Vox:

     Does the frontrunner have a glass jaw?
     All year, Biden has led in the vast majority of national and early-state polls of Democratic voters.

• glass chin, bad chin

• chin

• chinny (adjective)

• Jonathon Green (2024) “glass jaw n.”, in Green’s Dictionary of Slang
• Eric Partridge (2005) “glass jaw”, in Tom Dalzell and Terry Victor, editors, The New Partridge Dictionary of Slang and Unconventional English, volumes 1 (A–I), London, New York, N.Y.: Routledge, →ISBN, page 874.