"답정남" == 답이 정해저 있는 남자.
답이 정해저 있는 남자 => "A guy(남자) with a fixed (정해져 있는) response(답).
It means someone who always wants to say the same thing - i.e, fixated on certain idea (like how important he is, for example) and tries to drive the conversation in that direction.
It is one of those slang abbreviations recently coined and used mostly on the internet.
Not many people use, or even understand it - so no need to worry about such terms too much.