This English expression is used only in informal contexts and does not, as it may appear, relate to the French language in any way whatsoever. The word “excuse” is used interchangeably with the word “pardon” in the expression.
The phrase is generally said after someone has sworn or said something inappropriate. It is used as a way of coyly suggesting that they haven’t actually sworn and instead have said a word in French despite the fact that both parties are aware of the fact that this is not the case.
The first record of the expression used in the same way it is now dates to the year 1936 in the novel All Trees were Green by Michael Harrison. It reads “a bloody sight better (pardon the French!) than most.”
The expression however is said to date back to the 19th century. At this time well-educated English speakers often used French words and would then apologise for having done this as many of those listening would not understand the word. An example of this usage can be found in the 1830 edition of The Lady’s Magazine which reads “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.”
There is no consensus regarding why or when exactly the meaning of the phrase changed and became linked with the act of swearing.
An example of the use of this expression can be found in the 1980 film Raging Bull. In this film we find the line “please excuse my French. I’m gonna make him suffer. I’m gonna make his mother wish she never had him…”