"sleeper" in the noun sense
1. sleeper, slumberer
a rester who is sleeping
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
an unexpected achiever of success
"the winner was a true sleeper
4. tie, railroad tie, crosstie, sleeper
one of the cross braces that support the rails on a railway track
"the British call a railroad tie a sleeper"
5. sleeping car, sleeper, wagon-lit
a passenger car that has berths for sleeping
pajamas with feet worn by children
a piece of furniture that can be opened up into a bed
8. sleeper, sleeper goby
tropical fish that resembles a goby and rests quietly on the bottom in shallow water
an unexpected hit
"that movie was the sleeper of the summer"
Source: WordNet® (An amazing lexical database of English)
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