What does it mean when your 9 year old girl likes to wear boy clothes only?

not very much, many Girl's like being tomboys before puberty and like sports and wearing more boyish clothes, this doesn't mean they won't be a very girlly teenaged girl or woman, it could equally be who she is forever, she could be that kind of girl that likes beer and football and hanging with the guys, there's no way to know, the only thing to do is let it be, and be a good listener to see if there's anything she wants to tell you