unfitness vs ineptness

unfitness and ineptness both are nouns.

Word	Noun	Adjective	Verb	Adverb
unfitness	Yes	No	No	No
ineptness	Yes	No	No	No

As nouns, ineptness is a hypernym of unfitness; that is, ineptness is a word with a broader meaning than unfitness:

unfitness: the quality of not being suitable
ineptness: the quality of having the wrong properties for a specific purpose

Other hypernyms of unfitness include unsuitability, unsuitableness.

unfitness (noun)	ineptness (noun)
the quality of not being suitable	the quality of having the wrong properties for a specific purpose
lacking the power to perform	unskillfulness resulting from a lack of training
poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)