1620s, "abatement" (a sense now obsolete), alteration of Middle French descompte (16c., Modern French décompte), from Medieval Latin discomputus (source of Italian disconto), from discomputare, from dis- (see dis-) + computare "to count" (see compute). Commercial meaning "deduction for early or prompt payment" is from 1680s; meaning "a reduction in the price of goods" attested by 1837.
1620s, "reckon as an abatement or deduction" (a sense now obsolete), from Old French desconter "reckon off, account back" (13c., Modern French décompter), from Medieval Latin discomputare, from dis- "away, from" (see dis-) + computare "to reckon, to count" (see compute). Hence, "to abate, deduct" (1650s), and figurative sense "to leave out of account, disregard" (1702). Formerly also discompt. Commercial sense of "make a deduction from, put a reduced price upon" is by 1977. Related: Discounted; discounting.