Origin of uncle
Word Origin for uncle
late 13c., from Old French oncle, from Latin avunculus "mother's brother," literally "little grandfather," diminutive of avus "grandfather," from PIE root *awo- "grandfather, adult male relative other than one's father" (cf. Armenian hav "grandfather," Lithuanian avynas "maternal uncle," Old Church Slavonic uji "uncle," Welsh ewythr "uncle").
Replaced Old English eam (usually maternal; paternal uncle was fædera), which represents the Germanic form of the root (cf. Dutch oom, Old High German oheim "maternal uncle," German Ohm "uncle").
Also from French are German, Danish, Swedish onkel. First record of Dutch uncle (and his blunt, stern, benevolent advice) is from 1838; Welsh uncle (1747) was the first cousin of one's parent. To say uncle as a sign of submission in a fight is North American, attested from 1909, of uncertain signification.
say (cry) uncle
To admit defeat, to surrender: “Wilbur held his little brother in a headlock until he had to cry uncle.”
Also, say uncle. Concede defeat, as in The Serbs want the Bosnians to cry uncle, or If you say uncle right now, I'll let you go first in the next game. This phrase originated about 1900 as an imperative among school-children who would say, “Cry uncle when you've had enough (of a beating).” By the mid-1900s it was being used figuratively, as in the examples.
see cry uncle; Dutch uncle.