The term 'sleeper fastener' refers to a type of SNAP, which is a four-letter word. A SNAP is a type of closure used in clothing and other items, consisting of two interlocking discs that are held together by a spring. It is commonly used in garments such as jackets, shirts, and pants, as well as in accessories such as handbags and wallets. The term 'sleeper fastener' is used to describe a SNAP that is designed to be hidden from view, usually by being sewn into the fabric of the item. This type of SNAP is often used in garments that require a more discreet closure, such as pajamas or lingerie.