unfitness vs disability

unfitness and disability both are nouns.

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

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

unfitness: poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)
disability: the condition of being unable to perform as a consequence of physical or mental unfitness

unfitness (noun)	disability (noun)
the quality of not being suitable	the condition of being unable to perform as a consequence of physical or mental unfitness
lacking the power to perform
poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)