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.

A rescuer or deliverer. This term, which recalls the age of chivalry through the image of a dashing knight on horseback clad in polished armor, dates from the sixteenth century but has been in figurative use only since the mid-twentieth century. John Ciardi pointed out that the phrase has been used with two meanings: the “Mr. Right” of a young girl’s dreams, rescuing her from the humdrum with the promise of romance, and in politics, the idealistic reformer. One might add a third, the white knight of the modern-day corporation, who rescues the company from a hostile raider and averts an unwanted takeover. Quite figuratively, the poet William Rose Benét wrote, “Like a knight in glittering armor, Laughter stood up at his side” (“The Last Ally”).