Etymology

sleeper (n.)

Old English slæpere "one who sleeps, one who is inclined to sleep much," agent noun from sleep (v.). Meaning "strong horizontal beam" is from c. 1600. Meaning "dormant or inoperative thing" is from 1620s. Meaning "railroad sleeping car" is from 1875. Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in American English sports jargon, probably from earlier (1856) gambling slang sense of "unexpected winning card." Meaning "spy, enemy agent, terrorist etc. who remains undercover for a long time before attempting his purpose" first attested 1955, originally in reference to communist agents in the West.

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Definitions of sleeper

sleeper (n.)
a rester who is sleeping;
Synonyms: slumberer
sleeper (n.)
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;
sleeper (n.)
an unexpected achiever of success;
the winner was a true sleeper--no one expected him to get it
sleeper (n.)
one of the cross braces that support the rails on a railway track;
the British call a railroad tie a sleeper
Synonyms: tie / railroad tie / crosstie
sleeper (n.)
a passenger car that has berths for sleeping;
Synonyms: sleeping car / wagon-lit
sleeper (n.)
pajamas with feet; worn by children;
sleeper (n.)
a piece of furniture that can be opened up into a bed;
sleeper (n.)
tropical fish that resembles a goby and rests quietly on the bottom in shallow water;
Synonyms: sleeper goby
sleeper (n.)
an unexpected hit;
that movie was the sleeper of the summer
From wordnet.princeton.edu