Pardon the pun?? Pun meaning?

You can use a play on words whenever you wish. Puns were to the Greeks and Romans a high type of humor. In American English, they are typically seen as low-brow humor. So people ask you to forgive them for making a pun, as they don't want to be seen as unsophisticated. It is not so important nowadays, but about a century ago, and before, puns were inappropriate. That's why they say, "Pardon the pun".