I have come across it in the first episode of the sixth season of Friends. Here is the context:
Rachel: Well, I guess we just find a divorce lawyer? (Looks at Ross.)
Chandler: Well, I think, I think, Ross already has one. Now, this one’s free, right? Because you paid for the first two, so the third one’s free.
Ross: Laugh it up, but the joke’s on you. Because we don’t need to get divorced, okay? We we’re just gonna get an annulment.
I couldn't find any dictionary that gave the definition of laught it up. Is this some kind of non-standard American phrase? Could you tell me what it means and what is the synonymous phrase of it?