The story
From private + -eer probably after volunteer, shortened from the original term "private man-of-war".
First known use, by meaning
- from 17th c. A privately owned warship that acted under a letter of marque to attack enemy merchant ships and take possession of their cargo.
- from 17th c. An officer or any other member of the crew of such a ship; a government-sanctioned pirate.
- from 20th c. A private individual entrant into a race or competition who does not have the backing of a large, professional team.
- from 17th c. To function under official sanction permitting attacks on enemy shipping and seizing ship and cargo; to engage in government-sponsored piracy.
Etymology from Wiktionary, CC BY-SA 4.0.