406

Most likely if it's "406" on tshirt, it means a telephone area code, so person is from Montana. Less likely, and somewhat dated, it's from Zombieland, a 2009 American post-apocalyptic horror comedy film. The main actor lives in an apartment complex, and has a crush on a woman in apartment 406. She turns into a zombie, and his first encounter with a zombie. He eventually killed her with the lid the tank of his toilet. Very unlikely it's "406" the error code returned by an Internet server meaning Not Acceptable.