There are words that people think are rude now that didn't used to be rude...and there are words that people used to use that no one would understand any more.
Rude words are words that people in your society think are bad words to use.
It can be different in different families.
I remember the first guy I went out with when I was about 15. I went round to his place. His mum was a fairly elderly widow and I said something about this boy Brian being a greedy guts--which was to me quite an ordinary kind of thing to say.
Brian's mother was appalled, horrified. You don't use language like that, do you?
You have probably noticed that so-called 'really rude' words are usually associated with things that are not spoken about when people are being polite. These might include words to do with going to the toilet, or sex or religion.
As these are important things for people, it is possible that these words are stored in very deep, old parts of our brain--the parts of our brain that are more or less the same as in all mammals. So when we are angry or upset the 'old parts' of our brain get excited and out come the rude words.
Perhaps that's why people who are sort of permanently angry swear a lot.
This is only a theory of course. It hasn't been proved. But it's interesting that chimpanzees who have learned sign language sometimes use the sign language to 'swear' with.
Some humans have an inherited problem in their brain called Tourette's Syndrome. In some people with Tourette's the problem makes them access rude words from deep down and say them uncontrollably in incredibly embarrassing ways. People like this can't help it at all. It's a medical problem. There is a new drug that helps some of them.
I know an engineer with this problem. He is a lovely man but when he is working on something difficult and he has to concentrate he says, 'Fuck, fuck, fuck,' all the time. He can't help it.