Late 18th century; earliest use found in William Mitford (1744–1827), historian of ancient Greece. From ancient Greek πεντηκόντερος (also πεντηκόντορος) ship with fifty oars, use as noun (short for πεντηκόντερος ναῦς) of πεντηκόντερος, adjective from πεντήκοντα fifty.