clue

A variant spelling of clew, meaning a ball of thread, from Old English cliewen.

The meaning shifted through the myth of Theseus, who used a thread to retrace his steps out of the Minotaur's labyrinth. From there, clue came to mean anything that helps you find your way through a problem — the thread itself dropped out of everyday use entirely.

Clew survives today only as a sailing term for the corner of a sail; clue and clew are, at root, the very same word.