Origin of "Say uncle!"

Was just reading an older SD column about boxing scoring, and it mentioned the phrase “Calling ‘uncle’”. Where exactly did the practice of saying the word “Uncle” to signify that you accept defeat come from? Is it a bastardization of another word? Does it refer to an actual person?

From the Word Detective

It goes all the way back to the Roman Empire and the expression “Patrue, mi Patruissimo,” or “Uncle, my best Uncle”.

Oops. I beg your pardon; I thought I heard someone calling me.

Thus the bully’s demand: “Who’s your daddy?”