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" (source also of Armenian hav "grandfather," Hittite huhhas "grandfather," Lithuanian avynas "maternal uncle," Old Church Slavonic uji "uncle," Welsh ewythr "uncle"). Boutkan, however, says "the root probably denoted members of the family of the mother."
Replaced Old English eam (usually maternal; paternal uncle was fædera), which represents the Germanic form of the same root (source also of Dutch oom "uncle, grandfather, brother-in-law," Old High German oheim "maternal uncle, son of a sister" German Ohm "uncle," Old Norse afi "grandfather").
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.
1833, in biology, "reversion by influence of heredity to ancestral characteristics, resemblance of a given organism to some remote ancestor, return to an early or original type," from French atavisme, attested by 1820s, said to have been coined by French botanist Antoine-Nicolas Duchesne, from Latin atavus "ancestor, forefather," from at- perhaps here meaning "beyond" + avus "grandfather," from PIE *awo- "adult male relative other than the father" (see uncle).
"servile black man," 1922, somewhat inaccurately in reference to the humble, pious, but strong-willed main character in Harriet Beecher Stowe's novel, "Uncle Tom's Cabin" (1852). The image implied in the insult perhaps is more traceable to the late 19c. minstel show versions of the story, which reached a far wider audience than the book.
I don't recall anyone in the 1920s using the term 'Uncle Tom' as an epithet. But what's amazing is how fast it caught on (in the 1930s). Black scholars picked up (the term) and just started throwing it at each other. [Ernest Allen, quoted in Hamilton, Kendra, "The Strange Career of Uncle Tom," Black Issues in Higher Education, June 2002]
As a verb, attested from 1937.
"of or pertaining to an uncle," 1789, from Latin avunculus "maternal uncle," diminutive of avus (see uncle) + -ar. Used humorously for "of a pawnbroker" (uncle was slang for "pawnbroker" from c. 1600 through 19c.).
Being in genteel society, we would not, of course, hint that any one of our readers can remember so very low and humiliating a thing as the first visit to "my Uncle"—the first pawnbroker. We have been assured though, by those whose necessities have sometimes compelled them to resort, for assistance, to their avuncular relation, that the first visit—the primary pawning—can never be forgotten. [Household Words, May 15, 1852]
The extension of the sense to corresponding relationships of descent, "a generation younger than" (grandson, granddaughter) is from Elizabethan times. The inherited PIE root, *nepot- "grandchild" (see nephew) has shifted to "nephew; niece" in English and other languages (Spanish nieto, nieta). Old English used suna sunu ("son's son"), dohtor sunu ("son's daughter").