early 15c., "robbery upon the sea, the practice of robbing on the high seas," from Medieval Latin piratia, from classical Latin, Greek peirateia "piracy," from peiratēs "brigand, pirate" (see pirate (n.)).
Specifically, in the law of nations, the crime of depredations or wilful and aggressive destruction of life or property committed on the seas by persons having no commission or authority from any established state. As commonly used it implies something more than a simple theft with violence at sea, and includes something of the idea of general hostility to law. According to the opinion of some, it implies only unlawful interference with a vessel ; according to others, it includes also depredations on the coast by a force landing from the sea. [Century Dictionary]