Pardon my French

Inf. Excuse my use of swear words or taboo words. (Does not refer to real French.) Pardon my French, but this is a hell of a day. What she needs is a kick in the ass, if you'll excuse my French.

Please excuse my language. In the days when language propriety was more of an issue than it is now, using a word or phrase that was “unfit for mixed company” was likely to lead to embarrassment. Since French was considered a racy language, people excused themselves with “pardon my French.”