Why is it that before I let you out of that headlock I make you say "uncle" first?

@AstroChuck I don’t know what you’re thinking, but if you tried to put me in a headlock you’d be flying to the moon.

And here is a pretty good answer

The origin is traced to the Irish anacol, meaning an act of mercy or quarter. So it seems that it was brought over to America when the Irish immigrants came over and we just adopted that term.

I’d say no such thing. By strangling me you’d be doing me a favor.

Headlock???I thought we were dancing! LOL

Because you’re my mother’s brother.

I don’t know, uncles have a tendency to be a little more rough when playing around?

Because I’m a monkey.

Auntie doesn’t have the same ring..

You’re a cruel sadist, and Bob’s your uncle.

@earthduzt – WOW, great answer!!!!

here is a site that gives another explanation
http://www.wordorigins.org/index.php/site/say_uncle/

@PattyAtHome- Think I like that explanation the best.

It may be that the ritual is simply a way of making the victim call out for help from a grownup, thus proving his or her helplessness. Alternatively, it may have started as a way of forcing the victim to grant the bully a title of respect—in Roman times, your father’s brother was accorded nearly the same power and status as your father. The form of “uncle” used in the Latin phrase (“patrue”) tends to support this theory, inasmuch as it specifically denoted your paternal uncle, as opposed to the brother of your mother (“avunculus”), who occupied a somewhat lower rung in patrilineal Roman society.

from the Word Detective.com