To leave without saying good-bye. The British thought that sneaking away from a gathering without telling anyone you're going wasn't acceptable manners across the channel. Curiously, or perhaps typically, the French refer to the same practice as filer a` l'anglais (“take English leave”). Americans used to use the phrase without knowing its origin. It has been said that the French leave but never say good-bye, while Americans say good-bye but never leave. “French leave” is also military slang for deserting.

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.”