sleeper

(noun) a rester who is sleeping. Synonyms: slumberer.

(noun) a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal.

(noun) an unexpected achiever of success. Example: "The winner was a true sleeper--no one expected him to get it."

(noun) one of the cross braces that support the rails on a railway track. Synonyms: tie, railroad tie, crosstie. Example: "The british call a railroad tie a sleeper."

(noun) a passenger car that has berths for sleeping. Synonyms: sleeping car, wagon-lit.

(noun) pajamas with feet; worn by children.

(noun) a piece of furniture that can be opened up into a bed.

(noun) tropical fish that resembles a goby and rests quietly on the bottom in shallow water. Synonyms: sleeper goby.

(noun) an unexpected hit. Example: "That movie was the sleeper of the summer."