Where does using the phrase “pardon my French”, when swearing or using explicit language, originate from?
Le Merde!
It comes from the fact that French people are vulgarians and thus to swear is to behave like a personage of French extraction.
According to my French teacher, saying “pardon my French” is like making the implication that cursing is like a forgien language to you, it’s something you do so rarely. Why French and not German or Yiddish or Greek, I (and she) have no idea.
Then again, she really was French herself…so she might have just been trying to save face. 
[sub]God knows she accredited the French with inventing seemingly everything from silverware to the printing press…[/sub]
That should of been “La merde!”
I thought it was the exact opposite. that English was defined as being of the Vulgate language family, and when all of England was ruled by francophonic Norman aristocrats, everyone else was expect to speak French to them. So a germanic word like “shit” would be a no-no, but “merde” was okay. Saying “Pardon my French,” in that context meant “Sorry to use a Vulgar language, but I don’t know enough French to say what I’d like in your preferred language, m’lord.”
At least, thats the story my junior illustrated dictionary taught me as a youngster.