1) Poilus march off 2) Unauthorized departure 3) Unceremonious adieu 
French leave is `leave of absence without permission or without announcing one`s departure`, including leaving a party without bidding farewell to the host. The intent behind this behaviour is to leave without disturbing the host. The phrase is first recorded in 1771 and was born at a time when the English and French cultures were heavily inter......
French leave is slang for to leave without saying goodbye, to slip away unnoticed.
[n] - an abrupt and unannounced departure (without saying farewell)
[Treasure Island] an unauthorized, unnoticed, or unceremonious departure.
noun an abrupt and unannounced departure (without saying farewell)
a departure without ceremony, permission, or notice: Taking French leave, he evaded his creditors.