A phrase used to imitate a person who has just spoken. By saying it at someone, you'll feel and look better than the person you are "correcting".
But the phrase can also be good. Such as, you just finished making something that has never been made before. Then, another person uses assholish sarcasm to comment on your creation. The person insulting you is right to insult you because you didn't seem that excited about your creation.
An example of assholish sarcasm as being used by Person B.
Person A: *just finished making a gun out of paper* "Wow, pretty cool"
Person B: "Yeah it's pretty cool, it's nothing you know. I just finished making a gun out of paper, nothing too fancy you know."