tomboy

From tom +‎ boy. First attested in Ralph Roister Doister (published 1567, written circa 1552), where it is used to describe a boisterous girl; the OED says the citation is however "generally taken" to mean a boisterous boy, and says that a use in The Old Law (published 1656, thought to have been written circa 1599) "certainly" means a boy: "must young court-tits / play tomboys' tricks with her?" By 1579 it was attested in the meaning "an immodest woman", and by no later than 1592 it had developed its modern meaning of a “girl who acts like a boy”.