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?"

2. Metaphorically, a vulnerability of that sort.
   • 2019: "Post-debate polls show Biden’s lead shrinking and Harris gaining" by Gabriela Resto-Montero, 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. One obvious way the nomination process could play out is that the longtime frontrunner just, well, wins. But another possibility has been that once voters get a closer look at Biden on the campaign trail and on the debate stages, he won’t fare so well — and that some of his 24 rivals might seem more impressive.

• glass chin, bad chin

• chin

• chinny (adj.)