Definitions for "Underboss"
an assistant or second-in-command to a chief (especially in a crime syndicate)
the second in command to the boss.
Underboss (Consiligneri) is a rank in the American and Italian Mafia. The Underboss is usually a family member, such as a son, that will take over the family if the don is sick or killed. In the American mafia, it is the second highest rank a member can achieve, the highest being boss.