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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. "[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 Only gradually superseded earlier Brother Jonathan (1776), largely through the popularization of the figure by cartoonist Thomas Nast.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.
A figure who stands for the government of the United States and for the United States itself. Uncle Sam — whose initials are the abbreviation of United States — is portrayed as an old man with a gray goatee who sports a top hat and Stars and Stripes clothing. During World War I and World War II, posters of Uncle Sam exhorted young men to join the armed forces. (Compare John Bull.)
[said to have originated during the War of 1812 when Samuel Wilson of Troy, New York, locally known as Uncle Sam, stamped US on supplies he provided for the government, and this was jocularly taken to be his own initials]