clue

Etymology

A later spelling of clew “ball of thread or yarn”,^[1][2] inherited from Middle English clewe,^[2][3] inherited from Old English clēowen (“ball, sphere; skein”),^[3] ultimately from Proto-Indo-European *glew- (“to ball up, clump together”).^[3]

The modern sense comes from the ball of thread that Ariadne gave Theseus to find his way out of the Labyrinth.^[4][2] The purely figurative sense “that which points the way” dates from the 1620s.^[2] The spelling -ue arose when Middle English -ew words from French were respelled -ue and the change spread to native words such as hue and true.^[1][2]

References

1. [1] [2] Simpson, J. A.; Weiner, E. S. C., eds. (1989), The Oxford English Dictionary, 2nd ed., Oxford: Clarendon Press
2. [1] [2] [3] [4] [5] Harper, Douglas, Online Etymology Dictionary, https://www.etymonline.com
3. [1] [2] [3] Wiktionary, the free dictionary, https://en.wiktionary.org
4. [1] The American Heritage Dictionary of the English Language, Boston: Houghton Mifflin