Etymology
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weakness (n.)

c. 1300, "quality of being weak," from weak + -ness. Meaning "a disadvantage, vulnerability" is from 1590s. That of "self-indulgent fondness" is from 1712; meaning "thing for which one has an indulgent fondness" is from 1822.

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myasthenia (n.)

"muscular weakness," 1856, medical Latin; see myo- "muscle" + asthenia "weakness." Related: Myasthenic.

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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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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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beriberi (n.)

also beri-beri, paralytic disease prevalent in much of India, 1703, literally "great weakness," intensifying reduplication of Sinhalese beri "weakness."

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imbecility (n.)

early 15c., imbecilite, "physical weakness, feebleness (of a body part), impotence," from Old French imbécillité and directly from Latin imbecillitatem (nominative imbecillitas) "weakness, feebleness, helplessness," from imbecillus "weak, feeble," of uncertain origin (see imbecile). "Weakness in mind" (as opposed to body) was a secondary sense in Latin but was not attested in English until 1620s.

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fragility (n.)

late 14c., "moral weakness," from Old French fragilité "debility, frailty" (12c.), from Latin fragilitatem (nominative fragilitas) "brittleness, weakness," from fragilis "brittle, easily broken," from root of frangere "to break" (from PIE root *bhreg- "to break"). Meaning "quality of being easily broken" is from late 15c.

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asthenia (n.)

"weakness, debility," 1788, medical Latin, from Greek astheneia "want of strength, weakness, feebleness, sickness; a sickness, a disease," from asthenes "weak, without strength, feeble," from a- "not, without" (see a- (3)) + sthenos "strength, power, ability, might," which is of uncertain origin, perhaps from PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness."

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senility (n.)

"old age, especially the weakness or imbecility due to old age," 1753, from senile + -ity.

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neurasthenia (n.)

"nervous exhaustion," 1854, medical Latin, from neur- (form of neuro- before a vowel) + asthenia "weakness" (see asthenia). Related: Neurasthenic.

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