Etymology
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ataractic (adj.)

1906, of persons, "calm, serene," from Latinized form of Greek ataraktos "not disturbed" (see ataraxia) + -ic. From 1955 of drugs, "inducing calmness."

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dispassionate (adj.)

1590s, of persons, "free from passions, calm, disposed;" 1640s, "not dictated by passion, impartial;" from dis- "the opposite of" + passionate. Related: Dispassionately.

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tranquil (adj.)
mid-15c., a back-formation from tranquility or else from Latin tranquillus "quiet, calm, still." Related: Tranquilly.
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placidity (n.)

"tranquility, peacefulness, quietness," 1610s, from Latin placiditatem (nominative placiditas), from placidus "peaceful, quiet, gentle, calm" (see placid).

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lenient (adj.)

1650s, "relaxing, soothing" (a sense now archaic), from French lenient, from Latin lenientem (nominative leniens), present participle of lenire "to soften, alleviate, allay; calm, soothe, pacify," from lenis "mild, gentle, calm," which probably is from a suffixed form of PIE root *‌‌lē- "to let go, slacken."

The usual modern sense of "mild, merciful" (of persons or actions) is first recorded 1787. In earlier use was lenitive, attested from early 15c. of medicines, 1610s of persons. Related: Leniently.

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mouton enrage (n.)

"A normally calm person who has become suddenly enraged or violent" [OED], 1932, from French mouton enragé, literally "angry sheep." See mutton + enrage.

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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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composed (adj.)

"calm, tranquil, free from disturbance or agitation," c. 1600, past-participle adjective from compose (v.). Earlier (1560s) "made up of parts." Related: Composedly; composedness.

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quiesce (v.)

"become quiet or calm, become silent," 1821, from Latin quiescere "to rest," from suffixed form of PIE root *kweie- "to rest, be quiet."

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still (n.2)
c. 1200, "a calm," from still (adj.). Sense of "quietness, the silent part" is from c. 1600 (in still of the night). Meaning "a photograph" (as distinguished from a motion picture) is attested from 1916.
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