Informal the U.S. (government or people), personified as a tall, spare man with chin whiskers, dressed in a red, white, and blue costume of swallow-tailed coat, striped trousers, and tall hat with a band of stars
extended < abbrev. U.S.
See Uncle Sam in American Heritage Dictionary 4
The government of the United States, often personified by a representation of a tall, thin man having a white beard and wearing a blue tailcoat, red-and-white-striped trousers, and a tall hat with a band of stars: “intent on giving states greater incentive to save both their dollars and Uncle Sam's”(New York Times).