infirmity vs sickliness

infirmity and sickliness both are nouns.

Word	Noun	Adjective	Verb	Adverb
infirmity	Yes	No	No	No
sickliness	Yes	No	No	No

As nouns, sickliness is a hyponym of infirmity; that is, sickliness is a word with a more specific, narrower meaning than infirmity:

infirmity: the state of being weak in health or body (especially from old age)
sickliness: The state or characteristic of weakness, incapacity, or physical distress due to poor health, especially of a chronic nature.

Other hyponyms of infirmity include asthenia, astheny, cachexia, cachexy, wasting.

infirmity (noun)	sickliness (noun)
the state of being weak in health or body (especially from old age)	The state or characteristic of weakness, incapacity, or physical distress due to poor health, especially of a chronic nature.