c. 1300 (mid-13c. as a surname), "a sea-robber, sea-plunderer, one who without authority and by violence seizes or interferes with the ship or property of another on the sea," especially one who is habitually engaged in such robbery or sails the seas for the robbery and plunder of merchant vessels, from Old French pirate and directly from Medieval Latin pirata "sailor, corsair, sea robber" (source also of Spanish, Italian pirata, Dutch piraat, German Pirat), from Greek peiratēs "brigand, pirate," literally "one who attacks" (ships), from peiran "to attack, make a hostile attempt on, try," from peira "trial, an attempt, attack," from PIE *per-ya-, suffixed form of root *per- (3) "to try, risk."
An Old English word for it was sæsceaða. Figurative sense of "plunderer, despoiler" is from late 15c. Meaning "one who takes another's work without permission" first recorded 1701; sense of "unlicensed radio broadcaster" (generally transmitting from a ship outside territorial waters) is from 1913.