incompetent vs sad sack

incompetent and sad sack both are nouns.

Word	Noun	Adjective	Verb	Adverb
incompetent	Yes	Yes	No	No
sad sack	Yes	No	No	No

As nouns, sad sack is a hyponym of incompetent; that is, sad sack is a word with a more specific, narrower meaning than incompetent:

Other hyponyms of incompetent include blunderer, botcher, bumbler, bungler, butcher, fuckup, fumbler, stumbler, slouch.

incompetent (noun)	sad sack (noun)
someone who is not competent to take effective action	someone who makes mistakes because of incompetence

incompetent (adjective)	sad sack (adjective)
not qualified or suited for a purpose
legally not qualified or sufficient
not doing a good job
not meeting requirements
showing lack of skill or aptitude