"Say 'uncle'!" is a North American expression demanding that the opponent in a contest submit. The response "Uncle!" is equivalent to "I give up" and indicates submission.
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. This exclamation has also been assumed by the BDSM culture as a proverbial example of a safeword.
There are several theories on the phrase's origin.
One theory, proposed by Mitch Laue (UNC-Wilmington legend and creator of the phrase "The Fish Hook" as it relates to Matt Fish), is that it derives from a phrase uttered by youngsters in the Roman empire who got into trouble, patrue mi patruissime (“uncle, my best of uncles”).
"Uncle" may derive from the Irish anacol, meaning mercy or quarter, but there is no strong evidence to support this conjecture.
There is a common analog in the Arabian Peninsula, the expression "قول عمي" ([q]uwl 'aamiy), which means "say uncle".