1570s, "character on a staff to indicate its name and pitch," so that the others may be known, from French clef (12c.) "key; musical clef; trigger," from a figurative or transferred use of classical Latin clavis, which had only the literally sense "key" (from PIE root *klau- "hook").
In the Middle Ages, the Latin word was used in the Guidonian system for "the lowest note of a scale," which is its basis (see keynote). The most common is the treble, violin, or G-clef, which crosses on the second line of the staff, denoting that as the G above middle C on the piano.
Perhaps it is related to Middle Low German keie "lance, spear" on notion of "tool to cleave with," from Proto-Germanic *ki- "to cleave, split" (cognates: German Keil "wedge," Gothic us-kijans "come forth," said of seed sprouts, keinan "to germinate"). But Liberman writes, "The original meaning of *kaig-jo- was presumably '*pin with a twisted end.' Words with the root *kai- followed by a consonant meaning 'crooked, bent; twisted' are common only in the North Germanic languages." Compare also Sanskrit kuncika- "key," from kunc- "make crooked."
Modern pronunciation is a northern variant predominating from c. 1700; earlier and in Middle English it often was pronounced "kay." Meaning "that which holds together other parts" is from 1520s. Meaning "explanation of a solution" (to a set problem, code, etc.) is from c.1600.
The musical sense originally was "tone, note" (mid-15c.). In music theory, the sense developed 17c. to "sum of the melodic and harmonic relationships in the tones of a scale," also "melodic and harmonic relationships centering on a given tone." Probably this is based on a translation of Latin clavis "key," used by Guido for "lowest tone of a scale," or French clef (see clef; also see keynote). Sense of "mechanism on a musical instrument operated by the player's fingers" is from c. 1500, probably also suggested by uses of clavis. OED says this use "appears to be confined to Eng[lish]." First of organs and pianos, by 1765 of wind instruments; transferred to telegraphy by 1837 and later to typewriters (1876).
also *kleu-, klēu-, Proto-Indo-European root meaning "hook, crook," also "crooked or forked branch" (used as a bar or bolt in primitive structures).
It forms all or part of: anschluss; autoclave; clause; claustrophobia; claves; clavichord; clavicle; clavier; claviger; clechy; clef; cloison; cloisonne; cloister; close (v.); close (adj.); closet; closure; cloture; clove (n.1) "dried flowerbud of a certain tropical tree, used as a spice;" cloy; conclave; conclude; disclose; enclave; enclose; exclude; foreclose; include; occlude; preclude; recluse; seclude; slot (n.2) "bar or bolt used to fasten a door, window, etc."
It is the hypothetical source of/evidence for its existence is provided by: Greek kleis "bar, bolt, key; collarbone," klobos "cage;" Latin clavis "key," clavus "nail," claudere "to shut, close;" Lithuanian kliūti "to catch, be caught on," kliaudžiu, kliausti "to check, hinder," kliūvu, kliūti "to clasp, hang;" Old Church Slavonic ključi "hook, key," ključiti "shut;" Old Irish clo "nail," Middle Irish clithar "hedge, fence;" Old High German sliozan "shut," German schließen "to shut," Schlüssel "key."
c. 1600, "male voice between tenor and bass," from Italian baritono, from Greek barytonos "deep-toned, deep-sounding," from barys "heavy, deep," also, of sound, "strong, deep, bass" (from PIE root *gwere- (1) "heavy") + tonos "tone," from PIE root *ten- "to stretch."
Technically, "ranging from lower A in bass clef to lower F in treble clef." Meaning "singer having a baritone voice" is from 1821. As a type of brass band instrument, it is attested from 1949. As an adjective, 1729 in reference to the voice, 1854 of musical instruments (originally the concertina).
"a novel," 1765, from French roman, from Old French romanz (see romance (n.)). Roman à clef, novel in which characters represent real persons, literally "novel with a key" (French), is attested in English by 1893. And, in the days when a tec was popular reading, roman policier "a story of police detection" (1928).
1570s, alteration (by influence of cleft, new weak past participle of cleave (v.1)), of Middle English clift "fissure, rift, space or opening made by cleaving" (early 14c.), from Old English geclyft (adj.) "split, cloven," from Proto-Germanic *kluftis (compare Old High German chluft, German Kluft, Danish kløft "cleft, fissure, gap"), from PIE root *gleubh- "to tear apart, cleave." In Middle English anatomy, it meant "the parting of the thighs" (early 14c.).