The expression THAT’S A GOOD ONE is used to say that sth is amusing because it isn’t true:
She says she was born in Brazil? That’s a good one.
So, you’re saying you can understand all languages, right? That’s a good one.
– She told me you would introduce me to the president.
– That’s a good one. I don’t know the man.
– If you help me, I’ll mention you in my will.
– That’s a good one. You have nothing to offer.
Here’s the video version of the article: