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Phrase(s): a chink in one’s armor
Fig. a special weakness that provides a means for attacking or impressing someone otherwise invulnerable. (Alludes to an opening in a suit of armor that allows a weapon to penetrate.) • Jane’s insecurity is the chink in her armor. • The boss seems mean, but the chink in his armor is that he is easily flattered.
McGrawhill's American Idioms And Phrasal Verbs