mid-14c., from Old French ton (13c.), 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, taut string," related to teinein "to stretch" (see tenet). Sense of "manner of speaking" is from c.1600. First reference to firmness of body is from 1660s.