Where does the expression “to take French leave” come from and What does it mean?

We use the expression “to take French leave” now to mean to take one’s departure secretly or without authorized permission; thus, a soldier may be said to take French leave if he surreptitiously absents himself from quarters.

But, despite the fact that the French counterpart of the expression is, “to withdraw as the English (filer a l’anglaise),” the origin of the expression is attributed to a custom that originated in France in the eighteenth century.

The Emily Post of that day ruled that a guest who had a pressing engagement elsewhere might with propriety leave the function which he was attending without going through the formality of seeking his host or hostess and making a ceremonious apology for his departure.

The latter, it felt, might lead to A general exodus of guests and be embarrassing to the host.