Qu'est-ce qu'un sleeper? Qu'est-ce qu'un sleeper? One that sleeps: a heavy sleeper who was not wakened by the burglar. A sleeping car.Children's pajamas, usually with legs that cover the feet. Often used in the plural. One that achieves unexpected recognition or success, as a racehorse or movie.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.A horizontal structural member on or near the ground that supports weight.Chiefly British A railroad crosstie.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.