grouch vs fire-eater

grouch and fire-eater both are nouns.

Word	Noun	Adjective	Verb	Adverb
grouch	Yes	No	Yes	No
fire-eater	Yes	No	No	No

As nouns, fire-eater is a hyponym of grouch; that is, fire-eater is a word with a more specific, narrower meaning than grouch:

Other hyponyms of grouch include crab, crabby person, hothead, misanthrope, misanthropist.

grouch (noun)	fire-eater (noun)
a bad-tempered person	a performer who pretends to swallow fire
	a belligerent grouch
	a member of a fire department who tries to extinguish fires

grouch (verb)	fire-eater (verb)
show one's unhappiness or critical attitude