the beauty of language (English at least) is that you can throw random words together and as long as people understand what you're trying to get across, it works.   Hell, even if it doesn't actually make any sense, it's still fun to say.   eg: "holy prancing puppy puddles that was a sick song"
sounds like you are taking yourself way to seriously in life and need to lighten up.
Besides all that, I'd say that "fucking" - having sex if you will -  is pretty damn legit.    Besides that though, the word "fuck" doesn't even have an actual meaning, so perhaps being "legit as fuck" means something is infact NOT legit.
Something to think about.    Stop taking shit so seriously.