Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., "disease, sickness; lack of capability, weakness," from Latin infirmitatem (nominative infirmitas) "want of strength, weakness, feebleness," noun of quality from infirmus (see infirm). Cf. Middle French infirmité, Old French enfermete.
infirmity in·fir·mi·ty (ĭn-fûr'mĭ-tē)
A bodily ailment or weakness, especially one brought on by old age.
A condition or disease producing weakness.
A failing or defect in a person's character.