The Greek god Bacchus was no teetotaler — he slung them back, big time. A bacchanalian party is a wild, wine-soaked, rowdy affair. Bacchanalian is used to describe any event that Bacchus would have enjoyed.
The Romans celebrated Bacchus with wine, songs, dances and more wine — not the kind of behavior you would expect from self-respecting adults (and probably not the kind of thing they would tell their kids about). Bacchanalian sounds like “back-and-nail” yen so you might want to think of it as a description of party where everyone has one drink too many and wants to give each other a back rub.