This might be misguiding, but the expression doesn't mean you actually want to fuck-that. Just be patient and it's gonna be clear in a moment....
Just imagine someone answering ''fuck that'' to some question you had or some request you made. Hmmm.. it's sounds coarse at first, maybe even confusing or shoking, but when you actually look into that expression, you'll see that ''fuck that'' is positive, careless and provocative fixing of boundary. It soon becomes a kind of careless-categorical NO or some kind of vulgar, yet affirmative performance of indifference. What you say ''fuck that'' to, is just one of the many instances of meaningless things you're totally over with.
-Let's go now!
-No, let's stay.
-Come on, let's go, now!
-Let's just stay for a moment and then we'll leave.
-Fuck that, I'm leaving!