Hello. I have no idea what "I fought it" means in the context below.
(A little girl kept asking the boy too many questions. So he had an idea to shut her up.)
Boy: A group of guys beat me up last night. They beat the crap out of me and took all my money.
Girl: Why'd they do that?
Boy: Because I kept asking them too many questions.
Girl: (thought for a minute) I would've stopped asking them stuff.
Boy: (I fought it, but a little smile pulled up at the corners of my mouth.) I don't think you would've.
Girl: Prob'ly not. I like talking.
"I fought it" means he tried to keep a straight face. In other words he tried to suppress the little smile that would (and probably did) tip the girl off to his ironic intent.