"답은 정해져있고 너는 대답만 해," meaning the answer I want to listen from you is already set, all you need to do is say it.
Situation: A recently has started working out and actually lost weight, so she wants to boast of her achievement.
A: 나 요즘 살 너무 찐거 같아 그렇지? (I think I've gained a lot of weight recently, haven't I?)
B: 무슨소리야! 완전 좋아보이는데. (Of course not! you look great.)
Origin of the expression is unknown because a lot of young Korean people enjoy shortening and abbreviating words and sentences.
Instead, understanding why the term is widely used is more important than creation of the expression itself.
Boasting of things such as your achievement and money is quite frowned upon in Korea because it has long been influenced by Confucianism, which highly values modesty.
In spite of that, some Korean people want to show off, so they found a way to do so by saying stupidly contradictory things about themselves.
For example, everyone can notice that a guy called Kim always studies hard. In a mid-term, he succeeds in getting straight A+s, so he is happy for the return from his hard work. He wants his friends to know his achievement but don't want to look like a boasting jerk. So he tells his friends, "I screwed up my final term, I should have studied more," showing them his exam results. Of course, that pisses off his friends whose grade looks like the prominent broadcaster, BBC.
The obvious intention of those acting 답정너 makes the listeners irritated.