Is most likely a reference to the 'M&M' murders that Tony Spilotro-The real-life Nicky Santoro (Joe Pesci's character)-was involved in. In 1963 Spilotro participated in what became known as the M&M murders.
Nicky Santoro: Give me the fuckin' name!
Tony Dogs: Ch-Charlie M.
Nicky Santoro: Charlie M?
Tony Dogs: Charlie M.
Nicky Santoro: Charlie M? You made me pop your fuckin' eye out of your head to protect that piece of shit? Charlie M? You dumb motherfucker!
Tony Dogs: Kill me.
Nicky Santoro: I'll kill you. You motherfucker you! Frankie, do him a fuckin' favor.
You hear a little girl, Frankie? Is that a little girl, Ace? Is that a little fuckin' girl? What happened to the fuckin' tough guy who told my friend to stick it up his fuckin' ass?
Nicky Santoro