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.
"to impart tone to," 1811, from tone (n.). Related: Toned; toning. To tone (something) down originally was in painting (1831); general sense of "reduce, moderate" is by 1847.
1888, agent noun from tone (v.). As a photography chemical, from 1920; in xerography, from 1954.
early 14c., "a musical sound," unexplained variant of tone (n.). From late 14c. as "a well-rounded succession of musical notes, an air, melody." Meaning "state of being in proper pitch" is from mid-15c.