it is a mid 17th century word that comes from the word "volunteer"

1664, originally private man of war (1646), from private, probably modeled on volunteer, buccaneer. http://www.etymonline.com/index.php?term=privateer Mid-17th century. After volunteer http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861736855 A privateer was a private warship authorized by a country's government by letters of marque to attack foreign shipping. Privateering is often described as a form of "legal" piracy. Strictly, a privateer was only entitled to attack enemy vessels during wartime. However, states often encouraged attacks on opposing powers while at peace, or on neutral vessels during time of war, blurring the line between privateering and piracy. Privateers were an accepted part of naval warfare from the 16th to the 19th centuries, authorised by all significant naval powers. The costs of commissioning privateers was borne by investors hoping to gain a significant return from prize money earned from enemy merchants. England, and later the United Kingdom, used privateers to great effect and suffered much from other nations' privateering. In the late 16th century, British ships cruised in the Caribbean and off the coast of Spain, trying to intercept treasure fleets from the Spanish Main. The English government felt this was justified by the Spanish Armada seizing the ships of Sir Francis Drake and Sir John Hawkins, who were trying to sell West African slaves to Spanish colonies, where that activity was illegal. At this early stage the idea of a regular navy (the Royal Navy, as distinct from the Merchant Navy) was not present, so there is little to distinguish this activity from regular naval warfare. The privateering certainly had the support of Elizabeth I, who on occasion lent ships to or bought shares in expeditions. Attacking Spanish ships was part of a policy of aggressive competition with Spain, and helped provoke the first Anglo-Spanish War. Capturing a Spanish treasure ship would enrich the Crown as well as strike a practical blow against Spanish domination of America. http://en.wikipedia.org/wiki/Privateer