down in the mouth vs grim

down in the mouth and grim both are adjectives.

Word	Noun	Adjective	Verb	Adverb
down in the mouth	No	Yes	No	No
grim	No	Yes	No	No

As adjectives, down in the mouth and grim are synonyms defined as:

Other synonyms of down in the mouth include blue, depressed, dispirited, down, downcast, downhearted, gloomy, low-spirited, low.

down in the mouth (adjective)	grim (adjective)
filled with melancholy and despondency	shockingly repellent; inspiring horror
	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