Word Origin & History
"metal piece that works a lock," from O.E. cæg, of unknown origin, with no certain cognates other than O.Fris. kei. Perhaps related to M.L.G. keie "lance, spear" on notion of "tool to cleave with," from P.Gmc. *ki- "to cleaver, split" (cf. Ger. Keil "wedge," Goth. us-kijans "come forth," said of
seed sprouts, keinan "to germinate"). Figurative sense of "that which serves to open or explain" was in O.E.; meaning "that which holds together other parts" is from 1523. Musical sense of "tone, note" is 15c., but modern sense of "scale" is 1590, probably from L. clavis or Fr. clef, from use in the Guidonian system for lowest note of a scale, which is its basis (cf. keynote). Also extended to "mechanism on a musical instrument" (c.1500).
"low island," 1697, from Sp. cayo "shoal, reef," from Taino cayo "small island;" spelling infl. by M.E. key "wharf" (1306), from O.Fr. kai "sand bank" (see quay