A tomboy, these days is what we call a girl that has the attributes of a boy. These are girls that wear cloths like boys and often are interested in what men are like sports something not many other women like. And they often befriend boys more often than girls but normally a boy has more boy friends than girls and vice versa. The 1500s term meant a girl who was rude and aggressive not necessarily one similar to a boy. Before that time it was a boy who was rude and not a girl. If you research a bit the habits of a tomcat you might understand what the tomboy attitude really is. In 1930, the meaning of the word changed it meant a girl who liked the company of other men, more specifically it meant prostitutes.
The anglo-saxon origins of the word are tumbere which meant to dance and tumble.