1911 Encyclopædia Britannica/Clue

CLUE, or Clew (O. Eng. cluwe), originally a ball of thread or wool, the thread of life, which, according to the fable, the Fates spin for every man. The ordinary figurative meaning, a piece of evidence leading to discovery, or a sign pointing to the right track, is derived from the story of Theseus, who was guided through the labyrinth by the ball of thread held by Ariadne.