A crossie is a girl or a boy who dresses and acts like the opposite sex. Such as a girl might cut her hair in a boy style and wear "boy" clothes, or a boy might grow out his hair and wear "girl" clothes.
Girl 1: Ha! Check out that chick right there! *points to Girl 2*
Girl 3: Oh em gee, check out her hair! It looks like a dude's! She looks like a crossie!
Girl 2: -_- Bitches. *walks away*
Girl 1: Ha, stupid crossie. *walks away with Girl 3*