What does What larks means in Great Expectations?

It is very similar to "who knew" or "I wonder" only in Joe's Words he uses that saying instead

Um, not really, I'm afraid.

"What larks" expresses the sentiment of "what a hilarious time we'll have" in this context. Joe says this when he tells Pip that when he's well, they'll go out for a ride together. And then, "what larks, Pip"

So the answer is akin to: "what fun we'll have", and not "who knew" or "I wonder"