frailness vs sickliness

frailness and sickliness both are nouns.

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

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

frailness: 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 frailness include asthenia, astheny, cachexia, cachexy, wasting.

frailness (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.