The T-shirt evolved from an undergarment used in the 19th century called “the union suit.” It was cut into separate top and bottom garments, with the top long enough to tuck under the waistband of the bottoms. This quickly became popular among miners, dockworkers, and farmers who preferred the lightweight fabric in hotter weather conditions.
The first "T-shirt" then surfaced in the United States when they were issued as an undergarment by the US Navy sometime between 1898 and 1913, during the Spanish-American War. It had a crewneck and short sleeves, and was meant to be worn as underwear under a uniform. Soon it became standard issue for all recruits in the army. The T-shirt got its iconic name from its shape resembling the letter "T." It soon became common practice for sailors and Marines to remove their uniform jacket, wearing only the undershirt, to work parties and in submarines and tropical climates.
During the Great Depression, the T-shirt became popular with workers in various industries known as the working proletariat. The T-shirt was durable, easily fitted, easily cleaned, and inexpensive, and for these reasons it became a clear favorite among young boys, too. The word T-shirt became part of American English by the 1920s, and appeared in the Merriam-Webster Dictionary. Following World War II, it was worn by Navy men as undergarments and it slowly became common to see veterans wearing casual T-shirts with their uniform trousers.