an intricate knot tied by Gordius, the king of Phrygia, and cut by the sword of Alexander the Great after he heard that whoever undid it would become ruler of Asia
any very difficult problem; insoluble in its own terms
discoloration of metal surface caused by oxidation
make dirty or spotty, as by exposure to air; also used metaphorically; "The silver was tarnished by the long exposure to the air"; "Her reputation was sullied after the affair with a married man"