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late 14c., "one who works only for hire," from Old French mercenaire "mercenary, hireling" (13c.) and directly from Latin mercenarius "one who does anything for pay," literally "hired, paid," from merces (genitive mercedis) "pay, reward, wages," from merx (see market (n.)).
1530s, from mercenary (n.), or in part from Latin mercenarius "hired, paid, serving for pay."