An exclamation combining the words Fuck and Shit, it can be used as a negative statement standing in for "fuck that shit", or in a positive sense "that's the fucking shit".
Typically this portmanteau of fucking shit is used to swear in the presence of those who would otherwise frown upon the use of profane utterances: teachers, parents, and obnoxious religious acquaintances.
Fhit, I'm out of here.
Oh man, that's the FHIT!
Fhit Jessica you're looking large, as in fat.