Etymology
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lair (n.)
Old English leger "act or place of lying down; bed, couch; illness; the grave," from Proto-Germanic *legraz (source also of Old Norse legr "the grave," also "nuptials" (both "a lying down"); Old Frisian leger "situation," Old Saxon legar "bed," Middle Dutch legher "act or place of lying down," Dutch leger "bed, camp," Old High German legar "bed, a lying down," German Lager "bed, lair, camp, storehouse," Gothic ligrs "place of lying"), from PIE root *legh- "to lie down, lay." Meaning "animal's den" is from early 15c. Essentially the same word as layer (n.), but more ancient and differentiated in sense.
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*legh- 
Proto-Indo-European root meaning "to lie down, lay."

It forms all or part of: allay; anlage; belay; beleaguer; bylaw; coverlet; fellow; lager; lair; law; lawful; lawless; lawsuit; lawyer; lay (v.) "to cause to lie or rest;" ledge; ledger; lees; lie (v.2) "rest horizontally;" litter; lochia; low (adj.) "not high;" outlaw; scofflaw; stalag; vorlage.

It is the hypothetical source of/evidence for its existence is provided by: Hittite laggari "falls, lies;" Greek lekhesthai "to lie down," legos "bed," lokhos "lying in wait, ambush," alokhos "bedfellow, wife;" Latin lectus "bed;" Old Church Slavonic lego "to lie down;" Lithuanian at-lagai "fallow land;" Old Irish laigim "I lie down," Irish luighe "couch, grave;" Old English licgan "be situated, have a specific position; remain; be at rest, lie down."
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stalag (n.)
"German POW camp," 1940, from German Stalag, short for stammlager "main camp," from Old High German stam "stem" (from Proto-Germanic *stamniz, from suffixed form of PIE root *sta- "to stand, make or be firm") + Lager "bed, lair, camp, storehouse," from Old High German legar "bed, lair," from Proto-Germanic *legraz, from PIE root *legh- "to lie down, lay."
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thalamus (n.)
plural thalami, 1753, "the receptacle of a flower," Modern Latin, from Latin thalamus "inner chamber, sleeping room" (hence, figuratively, "marriage, wedlock"), from Greek thalamos "inner chamber, bedroom," related to thalame "den, lair," tholos "vault, vaulted building." Used in English since 1756 of a part of the forebrain where a nerve appears to originate.
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beleaguer (v.)
1580s, "besiege, surround, blockade," literal and figurative, from Dutch or Low German belegeren "to besiege," from be- "around" (from Proto-Germanic *bi- "around, about;" see by) + legeren "to camp," from leger "bed, camp, army, lair," from Proto-Germanic *legraz-, from PIE *legh-ro-, suffixed form of root *legh- "to lie down, lay." A word from the Flemish Wars (cognates: Swedish belägra, Dutch belegeren "besiege," German Belagerung "siege"). Spelling influenced by league. Related: Beleaguered; beleaguering.
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couch (n.1)

mid-14c., "a bed," from Old French couche "a bed, lair" (12c.), from coucher "to lie down," from Latin collocare (see couch (v.)). From mid-15c. as "a long seat upon which one rests at full length." Traditionally, a couch has the head end only raised, and only half a back; a sofa has both ends raised and a full back; a settee is like a sofa but may be without arms; an ottoman has neither back nor arms, nor has a divan, the distinctive feature of which is that it goes against a wall.

As symbolic of a psychiatric treatment or psychoanalysis, by 1952. Couch potato first recorded 1979.

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rouse (v.)

mid-15c., rousen, intransitive, probably from Anglo-French or Old French reuser, ruser; Middle English Compendium compares 16c. French rousee "abrupt movement." Sometimes also said to be from Latin recusare "refuse, decline," with loss of the medial -c-. Originally in English a technical term in hawking, "to shaking the feathers of the body," but like many medieval hawking and hunting terms it is of obscure origin.

The sense of "cause game to rise from cover or lair" is from 1520s. The word became general from 16c. in the figurative, transitive, meaning "stir up, cause to start up by noise or clamor, provoke to activity; waken from torpor or inaction" (1580s); that of "to awaken, cause to start from slumber or repose" is recorded by 1590s. Related: Roused; rousing.

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laird (n.)
"landed proprietor or hereditary estate-holder in Scotland," mid-15c. (mid-13c. as a surname), Scottish and northern England dialectal variant of lord, from Middle English laverd (see lord (n.)). Related: Lairdship.
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brainstorm (n.)

also brain-storm, by 1861 as a colloquial term for "fit of acute delirious mania; sudden dethronement of reason and will under stress of strong emotion, usually accompanied by manifestations of violence," from brain (n.) + figurative use of storm (n.).

The sense of "brilliant idea, mental excitement, fit of mental application," is by 1934 and seems to have evolved from the earlier sense:

Modern radio broadcasting is replete with examples of the resourcefulness, daring and hair-trigger thinking of the men who handle the big news breaks and special programs for the networks — the "brainstorm boys" the announcers and engineers call them. Eye-witness accounts of federal agents surrounding a gang lair, word pictures of dust storms, stratosphere flights, floods and fires — these are but a few of the programs brought to radio audiences by the brainstorm squad. [Popular Mechanics, July 1936]

The verbal meaning "make a concerted attack on a problem, involving spontaneous ideas," is by 1947. Related: Brainstormed; brainstorming.

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start (v.)

Old English *steortian, *stiertan, Kentish variants of styrtan "to leap up" (attested only in Northumbrian past participle sturtende), from Proto-Germanic *stert- (source also of Old Frisian stirta "to fall, tumble," Middle Dutch sterten, Dutch storten "to rush, fall," Old High German sturzen, German stürzen "to hurl, throw, plunge"). According to Watkins, the notion is "move briskly, move swiftly," and the Proto-Germanic word is from PIE root *ster- (1) "stiff."

From "move or spring suddenly," sense evolved by c. 1300 to "awaken suddenly, flinch or recoil in alarm," and by 1660s to "cause to begin acting or operating." Meaning "begin to move, leave, depart" (without implication of suddenness) is from 1821. The connection probably is from sporting senses ("to force an animal from its lair," late 14c.). Transitive sense of "set in motion or action" is from 1670s; specifically as "to set (machinery) in action" from 1841.

Related: Started; starting. To start something "cause trouble" is 1915, American English colloquial. To start over "begin again" is from 1912. Starting-line in running is from 1855; starting-block in running first recorded 1937.

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