Etymology
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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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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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Quietism (n.)

a form of mysticism which consists in abnegation of all exercise of the will and purely passive meditation on God and divine things, 1680s, from Italian quietismo, literally "passiveness," from quieto "calm, at rest," from Latin quietus "free; calm, resting" (from PIE root *kweie- "to rest, be quiet").

Originally in reference to the mysticism of Miguel Molinos (1640-1697), Spanish priest in Rome, whose "Guida spirituale" was published 1675 and condemned by the Inquisition in 1685. Related: Quietist; Quietistic.

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halcyon (adj.)
"calm, quiet, peaceful," 1540s, in halcyon dayes (translating Latin alcyonei dies, Greek alkyonides hemerai), 14 days of calm weather at the winter solstice, when a mythical bird (also identified with the kingfisher) was said to breed in a nest floating on calm seas. The name of this fabulous bird is attested in Middle English as alcioun (late 14c.).

The name is from Latin halcyon, alcyon, from Greek halkyon, variant (perhaps a misspelling) of alkyon "kingfisher," a word of unknown origin. The explanation that this is from hals "sea; salt" (see halo-) + kyon "conceiving," present participle of kyein "to conceive," literally "to swell" (see cumulus) probably is ancient folk-etymology to explain a loan-word from a non-Indo-European language. Identified in mythology with Halcyone, daughter of Aeolus, who when widowed threw herself into the sea and became a kingfisher.
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Nembutal 

type of barbiturate, originally used to calm patients before anesthesia and operation, 1930, proprietary name of pentobarbitone sodium, formed from letters and syllables from N(a) "sodium" + full chemical name 5-ethyl-5-1-methylbutyl barbiturate.

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serene (adj.)

mid-15c., of a day, "clear, fair, calm," from Old French serein and directly from Latin serenus "peaceful, calm, clear, unclouded" (of weather); figuratively "cheerful, glad, tranquil"(from PIE root *ksero- "dry," source also of Greek xeros "dry, arid;" see xerasia).

In English, the word has been applied to persons, characters, etc. since 1630s: "tranquil, unruffled." Related: Serenely. Middle English also had serenous (mid-15c.), of places, "having clear, fair weather."

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decompress (v.)

"relieve or reduce pressure," by 1866, from de- + compress (v.). In early use especially "restore gradually to normal conditions  of air pressure." Figurative sense "become calm, relax" is by 1964. Related: Decompressed; decompressing.

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placatory (adj.)

"conciliatory, intended to placate or appease," 1630s, from Latin placatorius "pertaining to appeasing," from placat-, past-participle stem of placare "to calm, appease, quiet, soothe, assuage," causative of placere "to please" (see please).

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quietude (n.)

"rest, repose, quiet, tranquility," 1590s, from French quiétude (c. 1500) or directly from Late Latin quietudo, from Latin quietus "free; calm, resting" (from PIE root *kweie- "to rest, be quiet"). In the same sense quietness is attested from mid-15c.

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