softheaded vs screwball

softheaded is not a noun while screwball is a noun.

Word	Noun	Adjective	Verb	Adverb
softheaded	No	Yes	No	No
screwball	Yes	Yes	No	No

As adjectives, softheaded and screwball are synonyms defined as:

Other synonyms of softheaded include crazy, half-baked.

softheaded (noun)	screwball (noun)
	a pitch with reverse spin that curves toward the side of the plate from which it was thrown
	a whimsically eccentric person

softheaded (adjective)	screwball (adjective)
foolish; totally unsound	foolish; totally unsound