the metaphorical relative that will make good on one's previous shovel talk and/or other threats of violence
Ex: "If you break her heart, I'll have to send Uncle Guido after you..."
The protectorate of Italian hoods back in the day, undoubtedly affiliated with the Mafia. The guy who comes in the middle of the night to carry out unethical deeds.
"I ain't scared o' those Tony's."
"Wisen up, you don't want a visit from Uncle Guido, do you?"