Etymology
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sedate (adj.)

"calm, quiet, placid," usually of persons or temperaments, 1660s, from Latin sedatus "composed, moderate, quiet, tranquil," past participle of sedare "to settle, make calm," causative of sedere "to sit" (from PIE root *sed- (1) "to sit"). Related: Sedately; sedateness (1640s).

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serenade (n.)

1640s, "a musical performance at night in open air" (especially "one given by a lover under the window of his lady" [OED]), from French sérénade (16c.), from Italian serenata "an evening song," literally "calm sky," from sereno "the open air," noun use of sereno "clear, calm," from Latin serenus "peaceful, calm, serene" (see serene (adj.)). The sense was influenced by Italian sera "evening" (from Latin sera, fem. of serus "late"). The meaning "piece of instrumental music suitable for a serenade" is attested from 1728.

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sedation (n.)

early 15c. (Chauliac), sedacioun, "act or process of alleviation of pain;" 1540s, "act of making calm," from French sédation and directly from Latin sedationem (nominative sedatio) "a quieting, assuaging, a calming," noun of action from past-participle stem of sedare "to settle, make calm," causative of sedere "to sit" (from PIE root *sed- (1) "to sit").

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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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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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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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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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