Americans and People familiar with USA, know “Uncle Sam” is a national personification of the United States. Several years back when I first came across this term , I didn’t understand it but eventually figured it out from the context. Over the years many times when I came across the term, I was curious on its origin and official status but never bothered to check it out.
Today I took the effort to find out and got the answer from here & here. It dates from the War of 1812 and there are multiple theories behind the actual origin.