In my mind, the word stands for "successfully dishonest." To be a furbo is to pursue your self-interest (and only your self-interest) to the detriment of another, others, society, the world, possibly the cosmos (beating out karma). Violence is not entailed, but instead there is an instrumental use of thinking, solely for selfish ends. What is interesting is the connotation Italians give to the word, which wavers between the positive and the negative; whereas "cunning" is always negative in English. I strongly suspect that the wavering comes from the fact that Italians consider furbizia positive in themselves (all the while not recognizing it as such) and negative in others. Since furbizia is indeed parasitical, it cannot be generalized, as can be seen in areas such as Naples and its surroundings.
Variants include the diminutive furbetto (for someone who tries to be furbo in a minor way, such as getting ahead of you in line while you're distracted), the pejorative furbastro (an unlikable furbo who is prone to screwing up), and furbacchione (a likable rascal who doesn't do major damage).
(In the photo, veteran politician Giulio Andreotti, whom no one would describe as furbetto)