Etymology
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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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incubus (n.)

"imaginary being or demon, credited with causing nightmares, and, in male form, consorting with women in their sleep," c. 1200, from Late Latin incubus (Augustine), from Latin incubo "nightmare, one who lies down on (the sleeper)," from incubare "to lie upon" (see incubate). Plural is incubi. Compare succubus.

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dormouse (n.)

long-tailed Old World rodent noted for its state of semi-hibernation in winter, early 15c., possibly from Anglo-French *dormouse "tending to be dormant" (from stem of dormir "to sleep," see dormant), with the second element mistaken for mouse; or perhaps it is from a Middle English dialectal compound of mouse (n.) and French dormir. French dormeuse, fem. of dormeur "sleeper" is attested only from 17c.

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sheet (n.1)

[length of cloth] Old English sciete (West Saxon), scete (Mercian) "length of cloth, covering, napkin, towel, shroud," from Proto-Germanic *skautjon-, from *skauta- "project" (source also of Old Norse skaut, Gothic skauts "seam, hem of a garment;" Dutch schoot; German Schoß "bosom, lap"), from PIE root *skeud- "to shoot, chase, throw."

By mid-13c. as "large square or rectangular piece of linen or cotton spread over a bed next to the sleeper." The sense of "oblong or square piece of paper," especially one suitable for writing or printing on, is recorded by c. 1500; that of "any broad, flat, relatively thin surface" (of metal, open water, etc.) is from 1590s. Of a continuous sweep of falling rain from 1690s. Meaning "a newspaper" is recorded by 1749.

Sheet lightning, caused by cloud reflection, is attested from 1794; sheet music is from 1857. Between the sheets "in bed" (usually with sexual overtones) is attested from 1590s (played upon in "Much Ado"); to be white as a sheet is from 1751. The first element in sheet-anchor (late 15c.), one used only in emergencies, appears to be a different word, of unknown origin, perhaps with some connection to shoot (v.) on the notion of being "shot out."

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