Etymology
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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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Definitions of infirmity

infirmity (n.)
the state of being weak in health or body (especially from old age);
Synonyms: frailty / debility / feebleness / frailness / valetudinarianism
From wordnet.princeton.edu