shoot one's mouth off vs mouth

shoot one's mouth off is not a noun while mouth is a noun.

Word	Noun	Adjective	Verb	Adverb
shoot one's mouth off	No	No	Yes	No
mouth	Yes	No	Yes	No

As verbs, mouth is a hypernym of shoot one's mouth off; that is, mouth is a word with a broader meaning than shoot one's mouth off:

Other hypernyms of shoot one's mouth off include speak, talk, utter, verbalise, verbalize.

shoot one's mouth off (noun)	mouth (noun)
	the opening of a jar or bottle
	the externally visible part of the oral cavity on the face and the system of organs surrounding the opening
	the opening through which food is taken in and vocalizations emerge
	an impudent or insolent rejoinder
	the point where a stream issues into a larger body of water
	an opening that resembles a mouth (as of a cave or a gorge)
	a person conceived as a consumer of food
	a spokesperson (as a lawyer)

shoot one's mouth off (verb)	mouth (verb)
speak spontaneously and without restraint	express in speech
	articulate silently; form words with the lips only
	touch with the mouth