"tenor violin," 1797, from Italian viola, from Old Provençal viola, from Medieval Latin vitula "stringed instrument," perhaps from Vitula, Roman goddess of joy (see fiddle), or from related Latin verb vitulari "to exult, be joyful." Viola da gamba "bass viol" (1724) is from Italian, literally "a viola for the leg" (i.e. to hold between the legs).
spelling conventional in 15c.-17c. English to add emphasis to borrowed French nouns ending in stressed -on; also used to represent Italian -one, Spanish -ón; all from Latin -onem. Compare shalloon (1670s) for French chalon, a kind of material used for linings. The ending is used occasionally to form words in English, such as spittoon, quadroon, and some older ones no longer with us, such as shabberoon "disreputable person" (c. 1700).