Etymology
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quietus (n.)

"release or discharge from debt, a final clearing of accounts," 1530s, short for Medieval Latin phrase quietus est "he is quit," from quietus "free" (in Medieval Latin "free from war, debts, etc."), also "calm, resting" (from PIE root *kweie- "to rest, be quiet"). The full Latin phrase was used in English from early 15c. Hence, "death" (i.e. "final discharge"), c. 1600. Latin quies also was used for "the peace of death."

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sober (adj.)
mid-14c., "moderate in desires or actions, temperate, restrained," especially "abstaining from strong drink," also "calm, quiet, not overcome by emotion," from Old French sobre "decent; sober" (12c.), from Latin sobrius "not drunk, temperate, moderate, sensible," from a variant of se- "without" (see se-) + ebrius "drunk," of unknown origin. Meaning "not drunk at the moment" is from late 14c.; also "appropriately solemn, serious, not giddy." Related: Soberly; soberness. Sobersides "sedate, serious-minded person" is recorded from 1705.
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sedative (adj.)

in medicine, "tending to calm or soothe," early 15c. (Chauliac), sedatif, from Old French sedatif and directly from Medieval Latin sedativus "calming, allaying," from sedat-, past participle stem of Latin sedare, causative of sedere "to sit" (from PIE root *sed- (1) "to sit").

The noun derivative meaning "a sedative drug" is attested by 1797, short for sedative salt, etc.; earlier it was used in a figurative or non-medical sense (1785), "whatever soothes or allays."

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supplication (n.)
late 14c., from Old French suplicacion "humble request," from Latin supplicationem (nominative supplicatio) "a public prayer, thanksgiving day," noun of action from past participle stem of supplicare "to beg humbly" (in Old Latin as sub vos placo, "I entreat you"), from sub "under" (see sub-) + placare "to calm, appease, quiet, soothe, assuage," causative of placere "to please" (see please). In ancient Rome, a religious solemnity, especially in thanksgiving for a victory or in times of public danger.
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meditation (n.)

c. 1200, meditacioun, "contemplation; devout preoccupation; private devotions, prayer," from Old French meditacion "thought, reflection, study," and directly from Latin meditationem (nominative meditatio) "a thinking over, meditation," noun of action from past-participle stem of meditari "to meditate, think over, reflect, consider," from a frequentative form of PIE root *med- "take appropriate measures." Meaning "meditative discourse on a subject" is early 14c.; meaning "act of meditating, continuous calm thought upon some subject" is from late 14c. The Latin verb also had stronger senses: "plan, devise, practice, rehearse, study."

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phlegmatic (adj.)

mid-14c., fleumatik, "having the temperament formerly supposed to result from predominance of the bodily humor phlegm" (cool, calm, self-possessed, and in a pejorative sense, cold, dull, apathetic;) late 14c., "composed of phlegm (the bodily humor); containing phlegm," from Old French fleumatique (13c., Modern French flegmatique), from Late Latin phlegmaticus, from Greek phlegmatikos "abounding in phlegm" (see phlegm). Related: Phlegmatical; phlegmatically.

A verry flewmatike man is in the body lustles, heuy and slow. [Bartholomew Glanville, "De proprietatibus rerum," c. 1240, translated by John of Trevisa c. 1398]
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quittance (n.)

c. 1200, cwitance, quitaunce, "payment, compensation;" c. 1300, "a discharge from a debt or an obligation," from Old French quitance (Modern French quittance), from quiter "clear, establish one's innocence;" also transitive, "release, let go, relinquish, abandon" (12c.), from quite "free, clear, entire, at liberty; discharged; unmarried," from Medieval Latin quitus, quittus, from Latin quietus "free" (in Medieval Latin "free from war, debts, etc."), also "calm, resting" (from PIE root *kweie- "to rest, be quiet"). The Middle English word also is in part from Medieval Latin quittantia, a variant of quietantia.

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supple (adj.)
c. 1300, "soft, tender," from Old French souple, sople "pliant, flexible; humble, submissive" (12c.), from Gallo-Roman *supples, from Latin supplex "submissive, humbly begging, beseeching, kneeling in entreaty, suppliant," literally "bending, kneeling down," perhaps an altered form of *supplacos "humbly pleading, appeasing," from sub "under" (see sub-) + placare "to calm, appease, quiet, soothe, assuage," causative of placere "to please" (see please, and compare supplication).

Meaning "pliant" is from late 14c.; figurative sense of "artfully obsequious, capable of adapting oneself to the wishes and opinions of others" is from c. 1600. Supple-chapped (c. 1600) was used of a flatterer. Related: Suppleness.
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cyclone (n.)

1848, "extensive storm characterized by the revolution of air around a calm center in which the wind blows spirally around the center," coined by British East India Company official Henry Piddington to describe the devastating storm of December 1789 in Coringa, India; irregularly formed from a Latinized form of Greek kyklon "moving in a circle, whirling around," present participle of kykloun "move in a circle, whirl," from kyklos "circle" (from PIE root *kwel- (1) "revolve, move round"). Applied to tornadoes from 1856.

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*‌‌lē- 

Proto-Indo-European root meaning "to let go, slacken." 

It forms all or part of: alas; allegiance; lassitude; last (adj.) "following all others;" late; latter; lenient; lenitive; lenity; let (v.) "allow;" let (n.) "stoppage, obstruction;" liege.

It is the hypothetical source of/evidence for its existence is provided by: Greek ledein "to be weary;" Latin lenis "mild, gentle, calm," lassus "faint, weary;" Lithuanian lėnas "quiet, tranquil, tame, slow," leisti "to let, to let loose;" Old Church Slavonic lena "lazy," Old English læt "sluggish, slow," lætan "to leave behind."  

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