mercenary (n.)

late 14c., mercenarie, "one who works only for hire, one who has no higher motive to work than love of gain," 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 "wares, merchandise" (see market (n.)). Specifically "a professional soldier in foreign service" by mid-17c.

"working or acting for reward, serving only for gain," hence "resulting from sordid motives, ready to accept dishonorable gain," 1530s, from mercenary (n.), or in part from Latin mercenarius "hired, paid, serving for pay."