take French leave

1. To depart or absent oneself from some place or event without ceremony, permission, or announcement. The official story is that he's sick, but I think he's just taking French leave. As the evening wore on, we decided to take French leave and make our way home.

2. In the military, to desert one's unit. The sergeant is facing a court martial after it was discovered that he'd taken French leave just before the deadly operation.

take French leave

make an unannounced or unauthorized departure.

This expression stems from the custom prevalent in 18th-century France of leaving a reception or entertainment without saying goodbye to your host or hostess.

take French ˈleave

(British English, old-fashioned or humorous) leave your work, duty, etc. without permission; go away without telling anyone: I think I might take French leave this afternoon and go to the cinema.This idiom is said to refer to the eighteenth-century French custom of leaving a dinner or party without saying goodbye to the host or hostess.