"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 or tickling. 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.
"Uncle" may derive from the Irish anacol, meaning mercy or quarter, but there is no strong evidence to support this conjecture.