grim vs forbidding

grim is not a noun while forbidding is a noun.

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

As adjectives, grim and forbidding are synonyms defined as:

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

Other synonyms of grim include dour.

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

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