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general::
Phrase(s): have a glass jaw
Fig. to be susceptible to a knockout when struck on the head. (Said only of boxers who are frequently knocked down by a blow to the head.) • When the prizefighter was knocked out cold by a right to the chin in the first round, the newspapers said he had a glass jaw. • Once a fighter has a glass jaw, he’s finished as a boxer.
McGrawhill's American Idioms And Phrasal Verbs