Definition: a wild and indulgent party
This comes from Bacchus, the Roman god of wine and revelry.
In some context, it has a sexual subtext, used to describe and orgy or indulgent sexual performance.
Humorously, one of the top search results for the word is the Bacchanal Buffet at Caesar’s Palace in Las Vegas.
Why I Looked It Up
In “The Tomb,” a short story by H.P. Lovecraft:
One morning at breakfast I came close to disaster by declaiming in palpably liquorish accents an effusion of eighteenth-century Bacchanalian mirth…
In the story, the main character is slowly being taken over by a spirit during the overnight hours, and apparently speaks with the inflections of that spirit at breakfast the next morning, running the risk of being discovered.