C19: apparently a humorous interpretation of the letters stamped on army supply boxes during the War of 1812: US
Uncle Sam for learners of EnglishPowered by COBUILD (ʌŋkəl sæm)
Some people refer to the United States of America or its government as UncleSam. [mainly US [mainly, also British, journalism] ⇒ They are ready to defend themselves against Uncle Sam's imperialist policies. ⇒ For years we were taught that Uncle Sam and foreign investment were the problem.