The history of the American T-shirt goes back to World War I when American troops noticed European soldiers wearing a comfortable, lightweight cotton undershirt during the hot and humid summer days. The European soldiers dealt with the heat by stripping off to their undershirt while the American soldiers stifled in their hot woolen uniforms. The undershirt quickly caught on with the American. The "T-shirt" got its name because of its simple design resembling the letter T.
By the 1920s, the T-shirt had become an official word in American English and was included in the Merriam-Webster Dictionary. By World War II, both the U.S. Navy and army had included the T-shirt as a standard piece of underwear.