"For A Girl" is a phrase in which you may be good at something, but it's better than other girls
but is still weak. Guys usually use this phrase to look down
on girls and assume that guys are better at things than girls, or how guys have more of an athletic build means that the guy is better at something than the girl. Until the girl beats them at something then in this case the girl might say "you're good
at that for a boy" which has the same meaning but is less used.