You're a selfish, aren't you?
Is this grammatically correct, and is it how a native speaker would say it? The context is that person A continually acts in a selfish way, and now another person B (a friend or colleague) who has been around him for a long time asks this question in a situation where person A says something selfish.
This is not gramatically correct, no.
"a" implies a noun, e.g. "person," "woman," "worker."
"You are a selfish [type of person]" would work, e.g. "You're a selfish man, aren't you?"
If you do not want to use a noun (because, for example, you don't know what kind of person they are because you are talking over the internet) you can instead write "You're selfish, aren't you?"
As already pointed out, it's not grammatically correct.
However, I could see someone saying it in this fashion if they were upset enough to call someone out on their behavior but didn't want to use an explicit insult.
That is, normally the word I would expect following "selfish" would be an epithet such as git, bastard, or bitch. If the speaker didn't want to use such an insulting word directly, they might just leave it out and let it be implied.
It's not gramatically correct, and I don't think a native speaker would say that, even if it were fixed. It comes across as somewhat rude, but not particularly forceful because of the question. If someone was annoyed enough to call someone out for being selfish, he probably wouldn't phrase it in such a way.
First of all, to fix up the grammar, one would say, "You're selfish, aren't you?" The extra "a" in there is extraneous.
But, as I said before, phrasing this as a question sends mixed signals. You're showing your annoyance by calling someone selfish, but weakening your statement by turning it into a question. In real-life, a person would probably say something like "You're selfish," or "You're being selfish," or "Why are you so selfish?" But he wouldn't put the "aren't you?" part at the end.
