To leave a place or an event without permission or notice, without taking leave of the hosts

But as I was certain I should not be allowed to leave the enclosure, my only plan was to take French leave and slip out when nobody was watching (Robert Louis Stevenson – Treasure Island, chapter 22)

Ma, essendo io certo che non m'avrebbero permesso di lasciare il recinto, il mio unico mezzo era congedarmi all'inglese, e scappar via mentre nessuno mi badava

The party started out very nicely, but when some drunk guests started yelling and fighting bitterly over nothing, I took French leave. The next day I called and apologized to the host

La festa era cominciata molto piacevolmente, ma quando alcuni ospiti ubriachi hanno cominciato a urlare e litigare violentemente per un nonnulla, me la sono filata all'inglese. Il giorno dopo, però, ho telefonato per scusarmi con la padrona di casa

The term French leave, stems from a mid-18th century French custom of leaving a formal gathering without bidding the host or hostess good-bye