Who is "the Butcher out of A.C.?"

The only important thing to take away here is that Christopher and Silvio know who "a certain Butcher out of AC" is.

The viewer doesn't need to know the specifics. All they need to know is that there is someone nicknamed "Butcher" who clearly would infer that Christopher's movie is referencing him (and would take offense to that).

Secondly, it makes sense that Silvio doesn't reveal personal details of "the Butcher from AC". He's in the process of pointing out to Christopher that "the Butcher from AC" wouldn't take kindly to being publicized; so it'd be counterproductive for Silvio to publicize his personal information while explaining this to Christopher.

Thirdly, who says they know more about this person other than his nickname? Maybe they simply don't know any personal details.

He's most likely referencing the real-life mobster Tommy "The Butcher" Pitera. Tommy was a pretty savage associate of the Gambino and Bonnano crime families who enjoyed dismembering his victims.

There was no known character in the Soprano's universe that was ever introduced as "The Butcher", but the Sopranos show was loosely based on a real life crime family so it makes sense they would reference other real life mobsters..