infirmness

infirm +‎ -ness

infirmness (uncountable)

1. The quality of being infirm; feebleness or illness.
   • 1664, Robert Boyle, Experiments and Considerations Touching Colours

     For both which purposes I thought it requisite to do these two things ; the one , to set down some experiments , which by the help of the reflections and insinuations that attend them , may assist you to discover the infirmness and insufficiency both of the common Peripatetic doctrine, and of the now more applauded theory of the chymists about colour

• infirmness in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913