A tomboy is a girl who does the things people expect boys to do — climbing trees, playing football, getting muddy, wearing shorts and trainers instead of dresses. It is usually said warmly, even admiringly, and often by the girl's own family: 'She was a real tomboy at that age.' But you should hear the assumption hiding inside it. The word only exists because someone decided in advance which games belong to boys, so calling a girl a tomboy quietly says she has crossed a line. Younger speakers today are noticeably more careful with it; many would just say she likes sport, or she's not girly, and would not reach for a label at all. The reverse word for a boy, sissy, has never been kind, and that difference tells you something. The word is old — it goes back to the 1500s, when Tom was simply a stock name for an ordinary man, the same Tom you see in tomcat.