In the United States and Canada, the idiomatic expression "Say 'uncle'!" may be used as an imperative command to demand submission of one's opponent, such as during an informal wrestling match. Similarly, the exclamation "Uncle!" is an indication of submission – analogous to "I give up" – or it may be a cry for mercy, in such a game or match.
Although it is often regarded as an Americanism, there are at least two differing theories as to the true origin of the phrase: ancient Rome and 19th century England.
The Roman Empire theory says, Roman children, when beset by a bully, would be forced to say the Latin phrase, "Patrue, mi Patruissimo," or, "Uncle, my best Uncle," in order to surrender and be freed. 
There are various opinions as to the reason that Roman bullies forced their victims to "cry 'Uncle! '". It may be that it was simply a way of making the victim call out for help from a grownup, thus proving his or her weakness. Alternatively, it may have started as a way of forcing the victim to grant the victor a title of respect – as in "Call me Uncle!" — for in Roman times, one's father's brother was accorded nearly the same power and status as one's father. The chosen form of the Latin word for "uncle" ("patrue") tends to support this theory, in that it specifically denotes the paternal uncle – as opposed to the brother of one's mother ("avunculus"), who occupied a somewhat lower rung in patrilineal Roman society.
The 19th century England theory says it comes from an English joke about a bullied parrot being coaxed to address his owner's uncle.