Etymology
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frail (adj.)
mid-14c., "morally weak," from Old French fraile, frele "weak, frail, sickly, infirm" (12c., Modern French frêle), from Latin fragilis "easily broken" (from PIE root *bhreg- "to break"). It is the Frenchified form of fragile. Sense of "easily destroyed, liable to break" in English is from late 14c. The U.S. slang noun meaning "a woman" is attested from 1908; perhaps with awareness of Shakespeare's "Frailty, thy name is woman."
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*bhreg- 

Proto-Indo-European root meaning "to break."

It forms all or part of: anfractuous; Brabant; bracken; brake (n.1) "stopping device for a wheel;" brake (n.2) "kind of fern;" brash; breach; break; breccia; breeches; brioche; chamfer; defray; diffraction; fractal; fraction; fractious; fracture; fragile; fragility; fragment; frail; frangible; infraction; infringe; irrefragable; irrefrangible; naufragous; ossifrage; refract; refraction; refrain (n.); refrangible; sassafras; saxifrage; suffragan; suffrage.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit (giri)-bhraj "breaking-forth (out of the mountains);" Latin frangere "to break (something) in pieces, shatter, fracture;" Lithuanian braškėti "crash, crack;" Old Irish braigim "break wind;" Gothic brikan, Old English brecan "to break."

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infirmity (n.)

late 14c., infirmite, "disease, sickness; lack of capability, weakness," from Old French infirmité, enfermete "illness, sickness, disease; moral weakness," and directly from Latin infirmitatem (nominative infirmitas) "want of strength, weakness, feebleness," also "the weaker sex" [Lewis], noun of quality from infirmus "weak, frail" (see infirm). 

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infirmary (n.)
mid-15c., "sick bay in a monastery," formerly also enfermerie, also firmary, fermery, from Old French enfermerie "hospital" and directly from Medieval Latin infirmaria "a place for the infirm," from Latin infirmus "weak, frail," (see infirm). According to OED, the common name for a public hospital in 18c. England.
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fragile (adj.)
Origin and meaning of fragile

1510s, "liable to sin, morally weak;" c. 1600, "liable to break;" a back-formation from fragility, or else from French fragile (Old French fragele, 14c.), from Latin fragilis "easily broken," from root of frangere "to break" (from PIE root *bhreg- "to break"). Transferred sense of "of frail constitution" (of persons) is from 1858.

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infirm (adj.)

late 14c., "weak, unsound" (of things), from Latin infirmus "weak, frail, feeble, not strong or firm" (figuratively "superstitious, pusillanimous, inconstant"), from in- "not, opposite of" (see in- (1)) + firmus "strong; stable," figuratively "constant, trusty" (from suffixed form of PIE root *dher- "to hold firmly, support" ). Of persons, "not strong, unhealthy," first recorded c. 1600. As a noun from 1711.

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*gere- (1)
*gerə-, Proto-Indo-European root meaning "to grow old." It forms all or part of: geriatric; geriatrics; gerontocracy; gerontology.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit jara "old age," jarati "makes frail, causes to age;" Avestan zaurvan "old age;" Greek geron "old man;" Ossetic zarond "old man;" Armenian cer "old, old man."
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thrush (n.1)

type of songbird, Old English þræsce, variant of þrysce, from Proto-Germanic *thruskjon (source also of Old Norse þröstr, Norwegian trost, Old High German drosca), from PIE *trozdo- (source also of Latin turdus, Lithuanian strazdas "thrush," Middle Irish truid, Welsh drudwy "starling," Old Church Slavonic drozgu, Russian drozdu).

An aged thrush, frail, gaunt, and small,
      In blast-beruffled plume,
Had chosen thus to fling his soul
      Upon the growing gloom.
[Hardy, "The Darkling Thrush," Dec. 31, 1900]
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frailty (n.)
mid-14c., freylte, from Old French fraileté "frailty, weakness," from Latin fragilitatem (nominative fragilitas) "weakness, frailty," from fragilis "fragile" (see fragility). Related: Frailties.
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imbecile (adj.)

1540s, imbecille "weak, feeble" (especially in reference to the body), from French imbecile "weak, feeble" (15c.), from Latin imbecillus "weak, feeble," a word of uncertain origin.

The Latin word traditionally is said to mean "unsupported" or "without a walking stick" (Juvenal: imbecillis: quasi sine baculo), from assimilated form of in- "not, opposite of" + baculum "a stick" (see bacillus), but Century Dictionary finds that "improbable" and de Vaan finds it "very far-fetched" and adds "it seems to me that exactly the persons who can walk without a support are the stronger ones." There also are phonological objections.

Sense shifted to "mentally weak or incapable" from mid-18c. (compare frail, which in provincial English also could mean "mentally weak"). As a noun, "feeble-minded person," it is attested from 1802. Traditionally an adult with a mental age of roughly 6 to 9 (above an idiot but beneath a moron).

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