Clue as a verb (normally suffixed by in) was only adopted relatively recently, in 1934, but the noun for something which hints at something else has been around since the 1590s. In the olden days, this was spelled clew, a word which still exists today as a term for "hammock cord" and originally meant "ball of thread or yarn". What caused this string-like connection? It all traces to Greek mythology! We can derive the meaning from the tale of the Minotaur, where Ariadne gave a clew of thread to Theseus so he could find his way out of the Labyrinth after killing it- thus the connection. In Middle English, clew could also be spelled clewe, and in Old English it was cleowen or cliewen. In Proto-Germanic, it's reconstructed as deriving from a word sounding like klewo and meaning "ball", and in Proto-Indo-European it was glew, which meant "conglomerate", because things can conglomerate into a balls. It's pretty whimsical that philologists found all this out by examining linguistic clues!
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.