Sammy the Bull is like a nickname for someone who snitches, usually getting the accused into some serious form of trouble due to their grassing. Comes from Salvatore 'Sammy the Bull' Gravano, who grassed on John Gotti.
Mostly used to describe those who engage in illegal activity themselves, and then grass about their so called friends, so that they themselves don't get into any trouble.
"And the Sammy the Bull award goes to, Fat Joe." - Tony Yayo (Tattle Teller lyrics)