What is the correct meaning of- what the f*c*? Mind blowing? Head boggling? As if? 나귀, 엉덩이 hole?

There isn't really a "correct meaning," but I think it originally just meant sex. (Like, "We fucked last night.") However, the term has broadened to mean much 더 많이 than that. It can really mean whatever 당신 want. It's a noun (like "he's a dumb fuck"), a verb ("go fuck yourself"), an adjective ("it's a fucking hellhole")... It means anything 당신 want it to mean, and 당신 can pretty much use it wherever whenever. (Just not in front of your grandmother. It IS a curse word...)