ineptness vs unfitness

ineptness and unfitness both are nouns.

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

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

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

Other hyponyms of ineptness include inappropriateness, unworthiness, inconvenience, unbecomingness.

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