dunderhead vs poor fish

dunderhead and poor fish both are nouns.

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

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

dunderhead: 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 dunderhead include dolt, dullard, pillock, pudden-head, pudding head, stupe, stupid, stupid person.

dunderhead (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