

I would say the phrases are used as a mild, somewhat but not wholly jocose, apology for having just used (or being about to use) language that the listener might find somewhat improper. I don't think the phrases are usually mumbled, they don't have to be insincere, they don't have to follow (or precede) a blue streak, and the improper word or words do not have to have been uttered in the presence of the elderly, the young, or the uptight. I don't quite agree with your dictionary. But he might just say "Pardon my French." If he started talking to me about the book, he might feel embarrassed about not knowing how to pronounce the author's name and so say "Pardon my French." I think he would be more likely to say something like "I don't know if I'm pronouncing it right," or maybe "Is that how you pronounce it?" if he knows that I know French. Suppose a friend of mine who doesn't know French has just read a translation of a French book, and suppose my friend has never heard the author's name pronounced. The literal and original meaning of the two sentences is, as you said, something like "excuse me for my poor French." They could still be understood that way, given the proper context. I thoughtthese two phrases are intended to mean ∾xcuse me for my poor French. As a foreign learner of English, I wonder how come ∾xcuse myFrench and Pardon my French are phrases mumbled insincerely.
