This is where an American writer uses an expletive associated with "international" (that is to say, British and Commonwealth English), usually in an effort to make a character seem more salty and "regional".The reason I'm adding this is that sometimes when this happens the writer doesn't seem to realise the expression in question, no matter how many times we've heard it in real life, is still considered bad language.
In Britain, Australia, New Zealand (South Africa seems to have it's own rich slang vocab all to itself), words like "w@nker", "bugg*r", "to$$er" et al, while they might sound strange and amusing to American audiences, are still profanity. They are words you don't say on television as much as the more familiar "f*ck", "$h-t" and so on. That's not to say they're NEVER said, and "bugg*r" seems to be becoming more and more acceptable as a deadpan response to mishap, but they're usually not.
When these words are used in a work that features a lot of swearing anyway, there's no problem and no trope. However, when these words are in a piece where the language is otherwise fairly mild, and where the character in question isn't established as being otherwise, it creates a fair amount of dissonance for those who are used to hearing the word. It's as if Sergeant O'Hara on the 1960s "Batman" TV show talked like a Quentin Tarantino character for one line before reverting to "...as I live and breathe..".