A t-shirt was an item of clothing.
Whilst on the SS Madame de Pompadour with the Tenth Doctor and Rose Tyler, Mickey Smith wore a t-shirt with a picture of the Nintendo Entertainment System controller over the caption, "Know Your Roots". (TV: The Girl in the Fireplace)
After avoiding his own death at Lake Silencio, River Song bought the Eleventh Doctor a t-shirt saying "I died at Lake Silencio and all my wife got me was this lousy t-shirt". (PROSE: Houdini and The Space Cuckoos)