I'm really knackered
when I was growing up, this was considered to be the referring to being really tired after a good fucking.
So, when I was growing up, it was really quite rude!
There is a set of words which I would consider to be hmm, casual? words and wouldn't use in certain contexts. So as a teacher I wouldn't use them, and I wouldn't use them when talking to parents, or even when teaching adults, as I would consider them to be unprofessional (unless I deliberately used them for effect)
in this list would be crap, fart, damn, bugger, knackered and many others which I can;t think of.
Then there are swear words, which are different eg fuck, cunt, shit.
Interestingly though there are some quite ordinary words which I use with care, (and wouldn't use in the classroom) - stupid and idiot being 2. My kids are not allowed to use stupid, idiot or shut-up.