I'd just like to add in, however, that like I mentioned there are certain "accepted" idiomatic expressions that are very commonly used in English. "Bullshit!" is an expression of disbelief or denial, just like in Russian they say "shto ty!" or in Spanish they say "Que te pasa?"

There's sort of a fine line between what's "normal" slang and what's not. I understand your confusion between the two. I don't want to discourage your goal of understanding slang, just wanting to make sure you understood that the source of said slang is not always as reliable a source :)

Cheers!

-Nah-