1712, from French encore "still, yet, again, also, furthermore" (12c.), generally explained as being from Vulgar Latin phrase *hinc ad horam "from then to this hour," or (in) hanc horam "(to) this hour" (Italian ancora "again, still, yet" is said to be a French loan-word). As a noun, from 1763; as a verb, from 1748. Related: Encored.
Whenever any Gentlemen are particularly pleased with a Song, at their crying out Encore ... the Performer is so obliging as to sing it over again. [Steele, "Spectator" No. 314, 1712]
There appears to be no evidence that either the Fr. or It. word was ever similarly used in its native country. The corresponding word both in Fr. and It. is bis; in It. da capo was formerly used. [OED]