knucklehead vs poor fish

knucklehead and poor fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
knucklehead	Yes	No	No	No
poor fish	Yes	No	No	No

As nouns, poor fish is a hypernym of knucklehead; that is, poor fish is a word with a broader meaning than knucklehead:

knucklehead: a stupid person; these words are used to express a low opinion of someone's intelligence
poor fish: a person who is not very bright

Other hypernyms of knucklehead include dolt, dullard, pillock, pudden-head, pudding head, stupe, stupid, stupid person.

knucklehead (noun)	poor fish (noun)
a stupid person; these words are used to express a low opinion of someone's intelligence	a person who is not very bright