knucklehead vs pudding head

knucklehead and pudding head both are nouns.

Word	Noun	Adjective	Verb	Adverb
knucklehead	Yes	No	No	No
pudding head	Yes	No	No	No

As nouns, pudding head is a hypernym of knucklehead; that is, pudding head 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
pudding head: a person who is not very bright

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

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