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.