semitone (n.)

word-forming element of Latin origin meaning "half," also loosely, "part, partly; partial, almost; imperfect; twice," from Latin semi- "half" (before vowels often sem-, sometimes further reduced to se- before m-), from PIE *semi- "half" (source also of Sanskrit sami "half," Greek hēmi- "half," Old English sam-, Gothic sami- "half").

The Old English cognate, sam-, was used in such compounds as samhal "in poor health, weakly," literally "half-whole;" samsoden "half-cooked" ('half-sodden'), figuratively "stupid" (compare half-baked); samcucu "half-dead," etymologically "half-alive" (see quick (adj.)); and the lingering survivor, sandblind "dim-sighted" (q.v.).

The Latin element was common in formations from Late Latin, as in semi-gravis "half-drunk," semi-hora "half hour," semi-mortuus "half-dead," semi-nudus "half-naked," semi-vir "half-man, hermaphrodite."

The Latin-derived form in English has been active in forming native words since 15c. Semi-bousi "half-drunk" ('semi-boozy'), now obsolete, was among the earliest (c. 1400). As a noun, semi has variously been short for semi-detached house (by 1912), semi-trailer (by 1942), semi-final (by 1942).

mid-14c., "musical sound or note," from Old French ton "musical sound, speech, words" (13c.) and directly from Latin tonus "a sound, tone, accent," literally "stretching" (in Medieval Latin, a term peculiar to music), from Greek tonos "vocal pitch, raising of voice, accent, key in music," originally "a stretching, tightening, taut string," which is related to teinein "to stretch" (from PIE root *ten- "to stretch").

The sense of "manner of speaking" is from c. 1600. In reference to firmness of body, from 1660s. As "prevailing state of manners" from 1735; as "style in speaking or writing which reveals attitude" from 1765. Tone-deaf is by 1880; tone-poem by 1845.