Girl: What is abortion?
Man: Ask your brother.
Girl: But I don't have a brother!
Man: Exactly!
Girl: What is abortion?
Man: Ask your brother.
Girl: But I don't have a brother!
Man: Exactly!
Listen up, buttercup. This joke is dark. The dude is implying the girl doesn't have a brother because she was aborted. See, it's funny because it's sad, like your life. You probably think abortion is when you can't finish a mission in Fortnight.