From The Collaborative International Dictionary of English v.0.48 [gcide]:
Uncle \Un"cle\, n. [OE. uncle, OF. oncle, uncle, F. oncle, fr.
L. avunculus a maternal uncle, dim. of avus a grandfather;
akin to Lith. avynas uncle, Goth. aw? grandmother, Icel.
[=a]i great grandfather.]
1. The brother of one's father or mother; also applied to an
aunt's husband; -- the correlative of aunt in sex, and of
nephew and niece in relationship.
[1913 Webster]
2. A pawnbroker. [Slang] --Thackeray.
[1913 Webster]
3. An eldery man; -- used chiefly as a kindly or familiar
appellation, esp. (Southern U. S.) for a worthy old negro;
as, "Uncle Remus." [Colloq.]
Plain old uncle as he [Socrates] was, with his great
ears, -- an immense talker. --Emerson.
[Webster 1913 Suppl.]
My uncle, a pawnbroker. [Slang]
Uncle Sam, a humorous appellation given to the United
States Government. See Uncle Sam, in Dictionary of Noted
Names in Fiction.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
Uncle Sam
n 1: a personification of the United States government