mid-12c., multeplier, "to cause to become many," from Old French multiplier, mouteplier (12c.) "increase, get bigger; flourish; breed; extend, enrich," from Latin multiplicare "to increase," from multiplex (genitive multiplicis) "having many folds, many times as great in number," from comb. form of multus (see multi-) + -plex "-fold," from PIE *plek- "to plait" (see ply (v.1.)). Mathematical sense is attested from late 14c. Related: Multiplied; multiplying.