Etymology
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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.

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mercenary (adj.)

"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."

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hack (adj.)
"hired, mercenary," 1812, from hack (n.2).
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condottiere (n.)

"professional leader of a mercenary troop," 1794, from Italian condottiere, from condotto "to conduct," from Latin conducere "to lead or bring together" (see conduce).

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free-lance (n.)
also freelance, "medieval mercenary warrior," 1820 ("Ivanhoe"), from free (adj.) + lance (n.); apparently a coinage of Sir Walter Scott's. The description of them resembles that of the Italian condottieri. Figurative sense is from 1864; specifically of journalism by 1882.
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hireling (n.)
"one who works for hire," Old English hyrling; see hire (v.) + -ling. Now only disparaging, "one who acts only for mercenary motives," a sense that emerged late 16c. As an adjective by 1580s.
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brigand (n.)

c. 1400, also brigaunt, "lightly armed irregular foot-soldier," from Old French brigand (14c.), from Italian brigante "trooper, skirmisher, foot soldier," from brigare "to brawl, fight" (see brigade). Sense of "robber, freebooter, one who lives by pillaging" is earlier in English (late 14c.), reflecting the lack of distinction between professional mercenary armies and armed, organized criminals.

Probably then it was in the sense of skirmishers that the name of brigand was given to certain light-armed foot-soldiers, frequently mentioned by Froissart and his contemporaries. ... The passage from the sense of a light-armed soldier to that of a man pillaging on his own account, is easily understood. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]
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larceny (n.)
"theft; wrongful or fraudulent taking of the personal goods of another with felonious intent," late 15c., from Anglo-French larcin (late 13c.), Old French larrecin, larcin "theft, robbery" (11c.), from Latin latrocinium "robbery, freebooting, highway-robbery, piracy," from latro "robber, bandit," also "hireling, mercenary," ultimately from a Greek source akin to latron "pay, hire, wages," from a suffixed form of PIE root *le- (1) "to get" (source also of Greek latreia "worship, service paid to the gods, hired labor," latron "pay, hire," latris "servant, worshipper").

Perhaps with -y (3) added in English or else the word was altered by influence of burglary, felony. Formerly distinguished into grand larceny, involving property valued in excess of a stated amount, and petty larceny.
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purchase (n.)

c. 1300 (late 12c. in surnames), purchas, "acquisition, gain;" also, "something acquired or received, a possession; property, goods;" and especially "booty, spoil; goods gained by pillage or robbery" (to make purchase was "to seize by robbery"). Also "mercenary soldier, one who fights for booty." It is from Anglo-French purchace, Old French porchaz "acquisition, gain, profit; seizing, plunder; search pursuit, effort," from Anglo-French purchaser, Old French porchacier (see purchase (v.)).

From early 14c. as "endeavor, effort, exertion; instigation, contrivance;" late 14c. as "act of acquiring, procurement." The meaning "that which is bought" is from 1580s. The sense of "hold or position for advantageously applying power, firm hold by which power may be exerted" (1711) is extended from the nautical verb meaning "to haul or draw (especially by mechanical power)," often used in reference to hauling up anchors, attested from 1560s. Wif of purchase (early 14c.) was a term for "concubine."

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