chink in one’s armor, a

A vulnerable spot, a weakness. The term alludes to the medieval knight’s armor made of mail—interlinked rings of metal jointed at various points. When a crack, or chink, developed between the links or joints, he was less protected against a spear or arrow. The noun “chink” has been used figuratively for such a fissure since the 1600s, and the current term came soon afterward. See also ACHILLES’ HEEL.