sleeper

(slēˈpər)

1. One that sleeps: a heavy sleeper who was not wakened by the burglar.
2. A sleeping car.
3. Children's pajamas, usually with legs that cover the feet. Often used in the plural.
4. a. One that achieves unexpected recognition or success, as a racehorse or movie.
   b. A spy or saboteur who is planted in an enemy country and who lives unobtrusively as a citizen of that country until activated into clandestine operations by a prearranged signal.
5. A horizontal structural member on or near the ground that supports weight.
6. Chiefly British A railroad crosstie.
7. Any of various usually small marine and freshwater fishes of the family Eleotridae, related to the gobies but lacking a sucking disk and noted for their habit of lying immobile.