Common affectionate (or not-so affectionate) name for the federal government of the UnitedStates.  (Does not refer to state or local governments, nor does the term refer to the UnitedStates as a whole).  Sometimes refers to specific departments within the US Government, most notably the InternalRevenueService.

It is widely believed that the name refers to a meat-packer named Samuel Wilson, who provisioned the US Army during the War of 1812.  Barrels of meat marked "U.S". were shipped to the Army; critics of the war suggested that "U.S." stood for "Uncle Sam" rather than "United States".

The image of UncleSam of a bearded guy in a red, white, and blue suit, was an subsequent invention of political cartoonists, most notably ThomasNast.  Samuel Wilson did not resemble UncleSam in any way.

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Contrast with UncleCharlie