The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

Uncle Sam
    n 1: a personification of the United States government