Owain Taggart wrote:
The message meant to say Good Fortune, but Rosetti was too insecure and paranoid, thinking it was something bad because he couldn't understand it. But being Italian, it's odd that he hadn't caught that. I think Nucky was being sincere with this, but you never know.
I think it's also a close analog to saying "good luck", which would explain why Rosetti lost his shit and decided to torch the big cop after he said that very phrase, as if it were an exacerbation of the initial perceived insult.