nut case vs fruitcake

nut case and fruitcake both are nouns.

Word	Noun	Adjective	Verb	Adverb
nut case	Yes	No	No	No
fruitcake	Yes	No	No	No

As nouns, nut case and fruitcake are synonyms defined as:

Other synonyms of nut case include crackpot, crank, nut, screwball.

nut case (noun)	fruitcake (noun)
a whimsically eccentric person	a rich cake containing dried fruit and nuts and citrus peel and so on
	a whimsically eccentric person