"Pardon My French"

Yesterday I was speaking with some friends whose wife and mother has just been diagnosed with incurable cancer. In one of those moments of not knowing quite what to say I came out with "Cancer sucks!" followed by "excuse my French". I try not to use words that would be considered obscene, and I realize that "sucks" is pretty low down on the cursing scale. But, well, cancer does suck.

Later I got to wondering about the phrase "excuse (or pardon) my French". It's sort of a strange thing to say when you think about it. A web search this morning turned up this google answers page where someone else asked the same question. The person answering the question listed several different answers she found on the web, but the jist of them seems to be that the French were (or are) considered vulgar and if you are using a curse word you are behaving as a French person would. Strange. Interestingly, in the comments, someone mentioned that they had understood it to come from the fact that the "f" word is the same word that the French use for seal. So if you said the "f" word then you would say "pardon my French" because it was in fact also a French word.

Here's how wordreference.com translates seal into French:
Principal Translations:
seal (adhesive for closing) nm sceau
seal (close, to) v fermer (enveloppe)
seal (animal) nm phoque

So, perhaps originally you only said "pardon my French" when using the "f" word and it eventually morphed into covering all curse words?