- sleeper (n.)
- early 13c., "one who sleeps, one who is inclined to sleep much," agent noun of sleep (v.). 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 gambling slang (1856) sense of "unexpected winning card." Meaning "spy, enemy agent, etc. who remains undercover for a long time before attempting his purpose" first attested 1955, originally in reference to communist agents in the West.