glass jaws

Noun

glass jaw (plural glass jaws)

1. 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).
   • "How was I to know that big ape had a glass jaw?"1989, Bryce Courtenay, The Power of One, →ISBN, page 109

   Synonyms:

   glass chin, bad chin

2. (figuratively) A vulnerability of that sort; a weak spot.
   • 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.1979, M. LeVan Rhame, Jon Chalmers Niemeyer, Tennis Magic: Playing With a Full Deck, New York: Vantage Press, page 164

   Synonyms:

   Achilles heel, chink in the armor, kryptonite, soft spot, soft underbelly, vulnerability, weakness, weak spot