A garment for the upper part of the body, typically having a collar, sleeves, and a front opening. An undershirt. A nightshirt. keep (one's) shirt on Slang To remain calm or patient: The plane doesn't land for another hour, so keep your shirt on. lose (one's) shirt Slang To lose everything one has or owns. the shirt off (one's) back Slang The maximum one is able to give or lose: The only thing those swindlers didn't take was the shirt off my back.