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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.
[entry form 1941+, second 1972+, third 1970s+; fr the late 1600s British sleasie, ''thin, flimsy, threadbare,'' of uncertain origin, whence it came to mean ''of inferior workmanship, shoddy''; perhaps fr Sleasie, ''Silesian,'' used of linen cloth from that part of Germany]
any of the marine and freshwater fishes of the family Eleotridae of the suborder Gobioidei (order Perciformes). Sleepers, found in warm and tropical regions, are so named because most species habitually lie quietly on the bottom. They are elongated fishes with two dorsal fins and are distinguished from most other gobies in having their pelvic fins separate, rather than joined to form a weak, rounded suction cup