Use this phrase to say that an idea is very incorrect.

For example, imagine that a friend says that you're not as good of a tennis player as you used to be. You disagree. The conversation goes like this:

A: We're all getting older, right? We can't play like we used to.

B: Oh, no. If anything, I'm actually better now than I was in college.

Person A thinks that you're a worse tennis player. You think that you're the same skill, or maybe even better than before.

Here's another example. Person A promised to go to Person B's art show, but now has to cancel:

A: I feel so bad for saying that I would go and then not following through.

B: No reason to feel bad. If anything, I'm the one who should feel bad for pressuring you into coming.

A: No, not at all! I really wanted to go.