Etymology
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uncle (n.)

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.

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ayah (n.)
"native nurse, children's governess," Anglo-Indian, 1782, from Portuguese aia, cognate with Spanish aya, Italian aja, etc., "nurse," from Latin avia "grandmother," fem. of avus "grandfather" (see uncle).
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atavism (n.)

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).

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Uncle Tom (n.)

"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.

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Uncle Sam (n.)
symbol of the United States of America, 1813, coined during the war with Britain as a contrast to John Bull, and no doubt suggested by the initials U.S. in abbreviations. "[L]ater statements connecting it with different government officials of the name of Samuel appear to be unfounded" [OED]. The common figure of Uncle Sam began to appear in political cartoons c. 1850. Only gradually superseded earlier Brother Jonathan (1776), largely through the popularization of the figure by cartoonist Thomas Nast. British in World War I sometimes called U.S. soldiers Sammies.
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avuncular (adj.)

"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]
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grand- 
a special use of grand (adj.) in genealogical compounds, originally with the sense of "a generation older than," first attested c. 1200, in Anglo-French graund dame "grandmother," also grandsire (late 13c.), from such use of Old French grand-, which perhaps is modeled on Latin avunculus magnus "great uncle." The partly-Englished grandmother, grandfather are from 15c. Other such words in European languages are formed with the adjectives for "old" or "best" (Danish bedstefar) or as diminutives or pet names (Greek pappos, Welsh taid). The French formation also is the model for such words in German and Dutch. Spanish abuelo is from Latin avus "grandfather" (from PIE *awo- "adult male relative other than the father;" see uncle), via Vulgar Latin *aviolus, a diminutive or adjective substitution for the noun.

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").
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uppity (adj.)
1880, American English, from up + -ity; originally used by blacks of other blacks felt to be too self-assertive (first recorded use is in "Uncle Remus").
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Abbassid 
dynasty of caliphs of Baghdad (C.E. 750-1258) claiming descent from Abbas (566-652), uncle of the Prophet. His name is from the same Semitic source as abbot. With patronymic suffix.
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