forbidding vs grim

forbidding is a noun but grim is not a noun.

Word	Noun	Adjective	Verb	Adverb
forbidding	Yes	Yes	No	No
grim	No	Yes	No	No

As adjectives, forbidding and grim are synonyms defined as:

forbidding and grim: harshly uninviting or formidable in manner or appearance

Other synonyms of forbidding include dour.

forbidding (noun)	grim (noun)
an official prohibition or edict against something

forbidding (adjective)	grim (adjective)
threatening or foreshadowing evil or tragic developments	shockingly repellent; inspiring horror
harshly uninviting or formidable in manner or appearance	filled with melancholy and despondency
	causing dejection
	not to be placated or appeased or moved by entreaty
	harshly ironic or sinister
	harshly uninviting or formidable in manner or appearance