It is not uncommon in languages for crudeness to serve as an indicator familiarity. In America one might jovially refer to a close friend as a "bastard", "motherfucker", or in certain subcultures use a word no white person except Eminem is allowed to use. I've heard many a young woman address her young female friends as 'cunt' or 'bitch'. Similarly, formality is often used to put distance between yourself and people you don't like, i.e. by calling them "sir".

Formality is used to put a respectful distance between people you don't like, but also people you don't know.

Too much familiarity with someone you don't known is a highly aggressive challenge. "What's up motherfucker" to some guy at the bar won't make any friends and might end in a fight.

As with many things there is quite a bit of nuance to the usage of registers, often differing by region. Opening conversation with strangers using an informal (but not crude) register is common in the US, and consequently the injection of formality can be done with hostile intention. If the hostility is not recognized by the other party the intention can be made clear by the juxtaposition of formality and crudeness. Consider a phrase like "Would you kindly fuck off?".

Eminem never says the N word tho. Quentin Tarantino comes to my mind as the famous white person with a pass.

"universally a negative + derogatory"

No it isn't. It's often hilarious, as in the above example. And in that one episode of Curb Your Enthusiasm. You wouldn't use it in mixed company, but if you know your audience you can say any number of horribly offensive things.

Indeed, thanks to the way culture is spreading through Youtube and other modern media, many here in America would prefer to adopt the lighthearted version of cunt from the aussies. Definitely not suggesting it's a majority though

Yep, I chose that example precisely because of the strong aversion to the word "cunt" in the US :)

Another example would be if I told a friend something incredulous, they might respond with "get tae fuck!" (meaning, "fuck off!"), or "awa' an shite!", (meaning "what a load of shit!").

How do you know when people are seriously cursing at you?

> How do you know when people are seriously cursing at you

Because there would be violence involved.

Seriously though, the tone, context and body language would be completely different.

It's hard to put it in words as our perception of aggression is a basic social instinct. But, broadly, you know from posturing (bucking up, moving in closer) eye contact, tone (more volume, or possibly less) and the cursing would get personal, demeaning, etc.

Context helps. A good friend is probably not cursing at you when you walk into the pub and he yells “how ya been, you old asshole?”

More interesting is how in the British Isles and Oceania, how cunt and mate have switched meaning.

I'd rather my friends call me a cunt (as the closest ones do) than mate, because the thin veneer of politeness in mate implies distance if not contempt (from strangers / acquaintances).

The Scots and the Australians use it in a much more conversational way than everybody else I've encountered.

It still has negative connotations, but in a much lesser way so it becomes part of friendly banter.