1 the part of a piece of clothing that covers your arm, or that covers part of your arm: long/short sleeves a dress with long sleeves → see also-sleeved2have something up your sleeve to have a secret plan or idea that you are going to use later: He still has a few tricks up his sleeve. → see also have an ace up your sleeveat ace1 (5)3a stiff paper cover that protects a book or a recordSYN jacket4a tube that surrounds a machine part [Origin: Old English sliefe] → see also, roll your sleeves upat roll up