If you do something selfishly, you do it in a way that looks after you and does not look after other people. The word is almost always a telling-off. Parents, teachers, and friends use it when someone takes the last piece, keeps the best seat, or talks only about their own plans. It is the adverb that belongs with selfish: you behave selfishly; you do not 'do selfish'. Though actually people sometimes joke with it — 'I'm going to selfishly take the afternoon off' — and then it is lighter, a bit of pretend guilt. In a serious sentence, though, it still stings. It is ordinary English, not slang and not high. You will hear it more when people talk about how someone acts than when they talk about things.