clue (n.)

A clue is something that helps you solve a puzzle. It comes from clew, an old word for a ball of thread.

Theseus went into the labyrinth holding one end of a thread. Ariadne held the other end at the entrance. He killed the Minotaur and found his way back out by following the thread.

That thread is why the word means what it means. A clue is the line you follow to get out of something.

He hadn’t a clue.