The question itself is self-explanatory! What's the meaning of it? And why is it named so? How was the anime named so? I thought the reason would be revealed by the end if the anime but it didn't, and I'm left here wondering...
As far as I remember the story is never set in Italy, however the Mafia-like organizations featured in the Anime are likely what makes the connection here and you will probably agree that there is quite a bit of "ruckus" going on,
It's called that because the book was. In the actual novel, The Narrator title drops the series in the second chapter, referring to events of 1930 (specifically the tale about the Liquor of Immortality) as a baccano (ruckus) when recounting the tale to a Japanese tourist in 2002 while they're waiting for Ronnie to recover a camera stolen by the Splot gang.
As for name itself, Narita had been watching some gangster flicks and got inspired to write a story about it with a few twists, and it ended up becoming a twisted tale of various subplots intertwining and colliding upon reaching the conclusion, and felt that the word baccano was an accurate descriptor of it while also fitting with the Mafia theme.