The white-bearded man in a top hat and stars-and-stripes suit who stands for the United States government, as on the 1917 army poster "I Want YOU". Chinese media also use the name for the United States itself.
The poster shows Uncle Sam pointing at the viewer.
Tips
history
The usual story credits Samuel Wilson, a meatpacker in Troy, New York, whose army rations in the War of 1812 were stamped 'U.S.', read as 'Uncle Sam'. But the nickname is already found in 1810, and the Wilson story first appeared in print in 1842.
memory
山姆 (Shānmǔ) is the standard transliteration of "Sam"; 大叔 ("uncle") translates the kinship part. The same 山姆 appears in 山姆会员店 - Sam's Club, Walmart's membership chain.