Yes, the "land of make believe" is an imaginary place where things are different from reality; they are what you wish were true, not what is real. So "In the land of make believe you're mine" means that I wish you were mine, but that's not reality. Re winning the match, I agree that "in your dreams" would be a better response, but "In the land of make believe" has a similar meaning: "In your imagination".
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NugsoAlso, would it be correct and/or natural if I(B) said:A: We're going to win this match.B: In the land of make believe.Not to me. I find these more idiomatic and usual.
CalifJimOn a rare occasion you'll hear When pigs fly, but that's a pretty old-fashioned expression.Oh, those expressions. I guess I can say when elephants fly, as well, right?
NugsoI guess I can say when elephants fly, as well, right?Yes, but then they'll know you're not American.