The unexpected turns a word can take between one use and another.
clue (n.)
Collection
Words That Changed Shape
A clue is something that helps you find an answer. It was once a clew, a ball of thread or yarn, from the Old English cliewen, a ball or skein.1
The change came from a Greek myth. Ariadne gave Theseus a thread to take into the Labyrinth, and once he had killed the Minotaur he followed it back out. Ovid tells the story in the Metamorphoses.2
From the 1590s English used the word for anything that guides a person through an intricate case, and by the 1620s for whatever points the way, with no labyrinth involved.1
The spelling changed as well. Middle English often wrote French words ending in -ue with -ew, and when their spelling went back to -ue, some native words followed, among them hue, true and clue. The spelling clue appears in the middle of the fifteenth century, and clew survived for the thread in northern English and Scots.1
There was no clue to who had written the letter.