scientific explanation:debauchery derives from the name of one ancient roman/greek deity called
Bacchus (lat.) or
Dionysus (gr). He was the patron of wine and vineyards (earlier), and agriculture and theater (later).
Since he was a deity, naturally, he had his worshipers who celebrated him in a rather specific manner: they would organize a wild and mystic festival, gathering involving excessive (wine) drinking and promiscuity.
So from that, the term
bacchanalia has been extended a bit, to refer to any drunken festivity, party.
And since the lovely french people are very famous for their vineyards and wine, they have adopted the term
bacchanalia, transforming it into term
débauche, with the same meaning... and as it's very well known that the lovely english people don't like the lovely french people, they just couldn't adopt the term
débauche as such, so they transformed it into
debauchery, with the identical meaning of course.
non-scientific explanation:debauchery is the thing when you call your friends (both male and female) over and tell them:
Yo folks, let's get loaded and let's get laid

I hope this was helpful
