clue
A clue today is a hint — the thing that points the way through a mystery, the lead the detective follows. The word is a respelling of clew, and a clew is a ball of thread. The figurative sense comes straight out of one story: the ball of thread Ariadne gave Theseus so he could find his way back out of the Labyrinth after killing the Minotaur. To follow a clue is, at the root, to follow a thread you have been paying out behind you — to find the way through a maze by holding onto a line. The spelling clue is older than the meaning: it was just an alternate spelling of clew by the mid-15th century, and only later did the two part ways — clew keeping the literal ball of yarn and the corner of a sail, clue taking the thread that leads you out of trouble.
A clue is a ball of thread. The word is a variant spelling of clew, the Old English cliewen, a wound ball of yarn — and the leap from "ball of yarn" to "hint that solves a mystery" runs through exactly one story. Ariadne, daughter of the king of Crete, gives Theseus a ball of thread before he enters the Labyrinth to kill the Minotaur. He ties one end at the door and unwinds it as he goes, and when the killing is done he follows the thread back out. The maze is unsolvable by reasoning — that is what a maze is — and solvable by a line you can hold. English took the name of that line and made it the name for anything that leads you out of an intricate place. By the 1590s a "clue" was what guided you through a labyrinth; by the 1620s the labyrinth had dropped away and a clue was simply what points the way.
The spelling came before the meaning. Clue is attested in the mid-15th century as just another way to write clew — the ball of thread — with no figurative sense attached. Only later did the two spellings divide the labor between them. Clew kept the literal ball of yarn, and the nautical sense (the lower corner of a sail, where the rope is made fast); clue walked off with the thread that leads through the maze. One Old English word for a ball of yarn became two modern words, and the abstract one forgot it had ever been concrete.
There is a tidier version of this etymology that you should not quite trust. In the popular telling, clew belongs to the same root as glue, gluten, and clay — all of them sticky, clumping things. Etymonline files them together under one root, *gleu-. But Wiktionary splits them: clew goes on *glew-, "to ball up, clump" (with cousins like Greek gloutós, "rump," a swelling, and Sanskrit gláu-, a tumor), while glue, gluten, and clay go on a separate root, *gleyH-, "to smear, stick." The two are related — both extensions of the same "clump" idea — but at the deepest layer they part, even as Wiktionary, which makes the split, still notes that clew is "akin to" the Old English word for clay. Whether a clue is the same word as glue depends on how deep you read. The thread holds; the glue is a thread you should not pull too hard on.
The false friends are easy to mistake for cousins. Clergy and cleric share the cl- opening but come from Greek klēros, "a lot, an allotment" — a portion drawn by chance, nothing to do with thread. Club and clump live in the same broad "clumping" semantic field but descend through a different Germanic formation. The ball of thread keeps only its true relations: clew itself, the Dutch kluwen, the German Knäuel. What the word still carries, underneath the detective and the mystery novel, is the oldest method for getting out of a place that reasoning cannot solve: not a map, but a line you laid down behind you, and the patience to wind it back in.
- clew — the same word, spelling diverged; it kept the literal "ball of thread or yarn" and the nautical "lower corner of a sail." Clue and clew are doublets — one lexeme split into two spellings with two jobs
- clew up / clew down — nautical, "to haul a sail up or down by its clews"; the rope-and-corner sense of the literal ball-of-cord word
- Knäuel — German, "ball of yarn, tangle"; from the same Proto-Germanic *klewô. Its initial Kn- arose by dissimilation in the diminutive — Old High German kliuwel became kniuwel, the first l shifting to n to break up the two l-sounds
- kluwen — Dutch, "ball of thread or yarn, clew"; a clean West Germanic cognate of the same ball-of-yarn word
- gloutós — Ancient Greek, "rump, buttock" — a swelling, a lump; the source of the anatomical gluteus. Wiktionary lists it under *glew-, the "ball up, swell" root, as a cousin of clew — though the deep Indo-European derivation is itself debated (Beekes allows a Pre-Greek origin)
- Cluedo / Clue — the board game (Britain, 1949; "Clue" in the US); the name is the figurative thread-that-solves-the-mystery word, used straight
- glue — contested root — from Latin gluten, "glue, beeswax," which Wiktionary places on *gleyH-, "to smear, stick" — a different root from clew's *glew-, "to ball up." Etymonline lumps them; Wiktionary splits them. The popular "a clue is the same word as glue" is true only under the lumping account
- clay / gluten — contested root — same split: Wiktionary derives them from *gleyH-, "to stick," not clew's "to ball up." Cousins of glue, not securely of clue
- clergy / cleric — different root — from Greek klēros, "a lot, an allotment, an inheritance," a portion drawn by chance; via Late Latin clericus. Shares only the cl- onset — no thread, no clump
- club / clump — different root — club is from Old Norse klubba; though both live in the broad *gel- "clump" field, neither is a confirmed cognate of the ball-of-thread word. A near-miss, not a cousin