What is French Leave?
1.
To leave or disappear without letting your party know you're leaving.
"Hey John, don't do a French Leave after work, we're all going to happy hour."
See
2.
Going AWOL, Desertion, etc.
"Zut alors! Où sont les militaires allés?"
"Ils ont pris, comme les Anglais disent, <<French Leave>>, quand ils ont entendu les gun-shots!"
"Hmm, comme d'habitude!" (Shrug of gallic insouciance).