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