hammerhead vs poor fish

hammerhead and poor fish both are nouns.

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

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

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

hammerhead (noun)	poor fish (noun)
medium-sized live-bearing shark with eyes at either end of a flattened hammer-shaped head; worldwide in warm waters; can be dangerous	a person who is not very bright
the striking part of a hammer
a stupid person; these words are used to express a low opinion of someone's intelligence