have a glass jaw
slang (of a boxer) To be apt to be knocked unconscious by one's opponent. Don't worry about that guy, he has a glass jaw—you'll have no trouble knocking him out.
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.