In the movie "Mean Girls", what does Regina George mean when she says "Shut up"?

Question

This "Best of Regina George" video includes several quotes from the movie Mean Girls, including the following conversation:

Regina: Wait. Sit down.
Seriously, sit down.
Why don't I know you?

Cady: I'm new. I just moved here from Africa.

Regina: What?

Cady: I used to be home-schooled.

Regina: Wait. What?

Cady: My mom taught me at home...

Regina: No, no.
I know what home-school is.
I'm not retarded.
So you've actually never been to a real school before?
Shut up.
Shut up.

Cady: I didn't say anything.

What does Regina mean by "Shut up"?

Answer 1

In this context, it is an exclamation of surprise and disbelief, like "Go on!" or "You don't say!" of yesteryear. If my sense of contemporary American pop culture is accurate, there's a gender bias, and "Shut up!" as an exclamation (not as a command to stop talking) is more often used by women than by men.

More generally, "shut up" also means "keep your mouth shut, i.e. stop talking", and that's why Cady says, "I didn't say anything."