uncle (n.) Look up uncle at Dictionary.com
late 13c., from Old French oncle, from Latin avunculus "mother's brother" ("father's brother" was patruus), literally "little grandfather," diminutive of avus "grandfather," from PIE root *awo- "grandfather, adult male relative other than one's father" (cognates: 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 (cognates: Dutch oom, Old High German oheim "maternal uncle," German Ohm "uncle").

Also from French are German, Danish, Swedish onkel. As a familiar title of address to an old man, attested by 1793; in the U.S. South, especially "a kindly title for a worthy old negro" [Century Dictionary]. First record of Dutch uncle (and his blunt, stern, benevolent advice) is from 1838; Welsh uncle (1747) was the male 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.