Etymology
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relax (v.)

late 14c., relaxen, "to make (something) less compact or dense" (transitive), originally especially in medicine, of muscles, etc., from Old French relaschier "set free; soften; reduce" (14c.) and directly from Latin relaxare "relax, loosen, open, stretch out, widen again; make loose," from re- "back" (see re-) + laxare "loosen," from laxus "loose" (from PIE root *sleg- "be slack, be languid"). A doublet of release.

Meaning "decrease tension" is from early 15c. From 1660s as "to make less severe or rigorous." Intransitive sense of "become loose or languid" is by 1762; that of "become less tense" is recorded from 1935. Of persons, "to become less formal," by 1837. Related: Relaxed; relaxing. As a noun, "relaxation, an act of relaxing," from 17c.

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relaxant (adj.)

1771, "causing or distinguished by relaxation," from Latin relaxantem (nominative relaxans), present participle of relaxare "to loosen, stretch out" (see relax). As a noun, "a medicine or treatment that relaxes or opens," from 1832. An earlier adjective was relaxative "having the quality of relaxing" (1610s).

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relaxation (n.)

late 14c., relaxacioun, "a rupture, a hernia" (a sense now obsolete); mid-15c., "remission of a burden or penalty," from Old French relaxacion (14c.) and directly from Latin relaxationem (nominative relaxatio) "an easing, mitigation, relaxation," noun of action from past-participle stem of relaxare "loosen, open, stretch out" (see relax).

Meaning "relief from hard work or ordinary cares; a state or occupation intended to give mental or bodily relief after effort or ordinary occupations and cares" is from 1540s. Sense of "remission or abatement of rigor or intensity" is from 1690s.

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*sleg- 

*slēg-, also *lēg-, Proto-Indo-European root meaning "be slack, be languid."

It forms all or part of: algolagnia; catalectic; laches; languid; languish; lax; lease; lessor; lush; relax; release; relish; slack (adj.); sleep.

It is the hypothetical source of/evidence for its existence is provided by: Greek legein "to leave off, stop," lagnein "to lust;" Latin languere "to be faint, weary," laxus "wide, spacious, roomy;" Old Church Slavonic slabu "lax, weak;" Lithuanian silpnas "weak."

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release (v.)

c. 1300, relēsen, "to withdraw, revoke (a decree, etc.), cancel, lift; remit," from Old French relaissier, relesser "to relinquish, quit, let go, leave behind, abandon, acquit," variant of relacher "release, relax," from Latin relaxare "loosen, stretch out" (from re- "back" (see re-) + laxare "loosen," from PIE root *sleg- "be slack, be languid"). Latin relaxare is the source also of Spanish relajar, Italian relassare, and English relax, and the uncle of relish.

Meaning "alleviate, ease" is mid-14c., as is sense of "set free from (duty, etc.); exonerate." From late 14c. as "grant remission, forgive; set free from imprisonment, military service, etc." Also "give up, relinquish, surrender." In law, c. 1400, "to grant a release of property." Of press reports, attested from 1904; of motion pictures from 1912; of music recordings from 1962. As a euphemism for "to dismiss, fire from a job" it is attested in American English since 1904. Related: Released; releasing.

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unbend (v.)

mid-13c., "relax a bow by unstringing it," from un- (2) "reverse, opposite of" + bend (v.). Intransitive sense from 1746. Figurative meaning "to become genial, relax" (1748) has a sense opposite to that of unbending "inflexible, obstinate" (1680s), which does not derive from the bow-stringing image.

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unloose (v.)

mid-14c., "relax;" late 14c., "to set free," from un- (2), used here emphatically, + loose (v.). Old English had unliesan "unloose, set free." Related: Unloosed; unloosing.

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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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unwind (v.)

early 14c., "to undo" (a bandage, wrapping, etc.), from un- (2) "opposite of" + wind (v.1). Similar formation in Old English unwindan "unveil, uncover," Dutch ontwinden, Old High German intwindan. Reflexive sense is recorded from 1740; figurative sense of "to release oneself from tensions, to relax" is by 1938. Related: Unwound; unwinding.

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elevator (n.)

1640s, originally of muscles which raise a part of the body, from Latin elevator "one who raises up," agent noun from past participle stem of elevare (see elevate). As a name for a mechanical lift (originally for grain) attested from 1787. Elevator music for bland, low-volume background music meant to relax listeners is attested by 1963. Elevator as a lift for shoes is from 1940.

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