The phrase appears in the U.S. in this usage as early as the 1800s, and linguists think that it derives from a more literal usage. That is, English speakers dropped French words or phrases into conversation—whether to display their culture, refinement or social class, or because sometimes only a French phrase has that certain je ne sais quoi—and then apologized for it if the listener wasn’t familiar with the word or didn’t speak the language. An example of this usage pops up in the 18th-19th century British fashion magazine The Lady’s Magazine, or Entertaining Companion for the Fair Sex, Appropriated Solely to Their Use and Amusement in 1830: “Bless me, how fat you are grown! Absolutely as round as a ball. You will soon be as enbon-point (excuse my French) as your poor dear father, the major.”

"Pardon my French" or "Excuse my French" is a common English language phrase ostensibly disguising profanity as words from the French language. The phrase is uttered in an attempt to excuse the user of profanity, swearing, or curses in the presence of those offended by it, under the pretense of the words being part of a foreign language.[1]

At least one source[2] suggests that the phrase "derives from a literal usage of the exclamation. In the 19th century, when English people used French expressions in conversation they often apologized for it – presumably because many of their listeners (then as now) wouldn't be familiar with the language".

The definition cites an example from The Lady's Magazine, 1830:[3] (passage cited in the prior reference.)