Etymology
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privateer (n.)

1660s, "private man of war, armed vessel owned and officered by private persons, usually acting under commission from the state," from private (adj.), probably on model of volunteer (n.), buccaneer. From 1670s as "one commanding or serving on a privateer." As a verb, 1660s (implied in privateering) "to cruise on a privateer, to seize or annoy an enemy's ships and commerce."

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dun (v.)

"to insist on payment of debt," 1620s, also as a noun, "agent employed to collect debts," of uncertain origin, perhaps related to Middle English dunnen "to sound, resound, make a din" (c. 1200, dialectal variant of din), or shortened from dunkirk (c. 1600) "privateer," a private vessel licensed to attack enemy ships during wartime, from Dunkirk, the French port from which they sailed. The oldest theory traces it to a Joe Dun, supposedly a London bailiff famous for catching defaulters. Related: Dunned; dunning. As a noun from 1620s.

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corsair (n.)

"government-sanctioned freebooter of the seas," 1540s, from French corsaire (15c.), from Provençal cursar, Italian corsaro, from Medieval Latin cursarius "pirate," from Latin cursus "course, a running," from currere "to run" (from PIE root *kers- "to run"). The sense of the Medieval Latin verb evolved from "course" to "journey" to "expedition" to an expedition specifically for plunder. As "a privateering pirate ship" from 1630s.

The name in the languages of the Mediterranean for a privateer; chiefly applied to the cruisers of Barbary, to whose attacks the ships and coasts of the Christian countries were incessantly exposed. In English often treated as identical with pirate, though the Saracen and Turkish corsairs were authorized and recognized by their own government as part of its settled policy towards Christianity. [OED]
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