An exclamation usually used to expressed surprise or disgust. The combination of bull and fucking to shit further adds to the emphasis of the intended meaning of shit. Similar in nature to Jesus Fucking Christ, Motherfucking Shit, or Chicken-Shit Motherfucker.

when you're hungover as shit after a spending a night drunk as a skunk

nonsense, bullshit, lies, crap, fools-gold, stupid-fodder, infuckingsane

absolute nonsence. Also used as an interjection to convey disgust and disbelief.

A word used to describe a situation or thing that is incredibly unfair and worse than bs.

Beyond bullshit. Used when some random person tells you a relationship ("she wants to be your girlfriend") or hook-up lie.

There are four types of bull:
Bull = dumb / rubbish
Bullcrap = a terrible situation
Bullshit = an absolute lie or threat
Bullfuckingshit* = more than an absolute lie; describes high school romantic rumors and lies

*Politer way to use if under 18: bullfreakingcrap OR bullfreakingshit