Origin: 1350–1400; Middle English mercenarie < Latin mercēnnārius working for pay, hired worker, mercenary, perhaps, representing earlier *mercēd(i)nārius, equivalent to *mercēdin-, stem of *mercēdō, a by-form of mercēs, stem mercēd- payment, wage (akin to merx goods; compare merchant) + -ārius-ary
late 14c., "one who works only for hire," from L. mercenarius "one who does anything for pay," lit. "hired, paid," from merces (gen. mercedis) "pay, reward, wages," from merx (see market). The adj. is recorded from 1530s.