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.