When a mean, harmful, or bad ass thing is done to someone the person who commits the act is a Dirty Girl.
When Clynt sniped Ethan, in call of duty MW2, while he was jumping through the air. Everyone said, "Oooohhh!!! That was dirty! Clynts a dirty girl!
When Will tackled John so hard, in football, that his helmet went flying off and he was unconscious for a whole minute. The team exclaimed that will was a Dirty Girl.