Etymology
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quiet (n.)

c. 1300, "freedom from disturbance or conflict; calm, stillness," from Old French quiete "rest, repose, tranquility" and directly from Latin quies (genitive quietis) "a lying still, rest, repose, peace" (from PIE root *kweie- "to rest, be quiet").

From late 14c. as "inactivity, rest, repose;" from c. 1400 as "absence of noise."

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quiet (adj.)

late 14c., "peaceable, being in a state of rest, restful, tranquil, not moving or agitated," from Old French quiet and directly from Latin quietus "calm, at rest, free from exertion," from quies (genitive quietis) "rest" (from PIE root *kweie- "to rest, be quiet").

From 1510s as "peaceable, not turbulent, characterized by absence of commotion." By 1590s as "making no noise." From 1570s as "private, secret." As an adverb from 1570s. Quiet American, frequently meaning a U.S. undercover agent or spy, is from the title of Graham Greene's 1955 novel. Related: Quietly; quietness.

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quiet (v.)

late 14c., "subdue (a sensation), lessen (a pain)," from quiet (adj.) and in part from Latin quietare. From mid-15c. as "to make or cause to be quiet;" intransitive sense of "become quiet, be silent" is from 1791 (with down (adv.) by 1851). Related: Quieted; quieting.

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unquiet (adj.)
1520s, from un- (1) "not" + quiet (adj.).
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quieten (v.)
1828, "to make quiet;" 1890, "to become quiet," from quiet (adj.) + -en (1).
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disquiet (v.)

"deprive of peace, rest, or tranquility," 1520s, from dis- + quiet (v.). Related: Disquieted; disquieting. As a noun, "want of quiet, rest, or peace," 1580s.

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*kweie- 
*kweiə-, also *kwyeə-, Proto-Indo-European root meaning "to rest, be quiet."

It forms all or part of: acquiesce; acquit; awhile; coy; quiesce; quiescent; quiet; quietism; quietude; quietus; quit; quitclaim; quite; quit-rent; quittance; requiescat; requiem; requite; while; whilom.

It is the hypothetical source of/evidence for its existence is provided by: Avestan shaitish "joy," shaiti- "well-being," shyata- "happy;" Old Persian šiyatish "joy;" Latin quies "rest, repose, quiet;" Old Church Slavonic po-koji "rest;" Old Norse hvild "rest."
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q.t. (n.)
slang for "quiet," in phrase on the q.t., attested from 1874. Phrase on the quiet appears from 1847.
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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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sedate (adj.)
"calm, quiet," 1660s, from Latin sedatus "composed, moderate, quiet, tranquil," past participle of sedare "to settle, calm," causative of sedere "to sit," from PIE root *sed- (1) "to sit." Related: Sedately.
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