mouth off vs spout

mouth off is not a noun while spout is a noun.

Word	Noun	Adjective	Verb	Adverb
mouth off	No	No	Yes	No
spout	Yes	No	Yes	No

As verbs, mouth off and spout are synonyms defined as:

Other synonyms of mouth off include jabber, rabbit on, rant, rave.

mouth off (noun)	spout (noun)
	an opening that allows the passage of liquids or grain

mouth off (verb)	spout (verb)
talk in a noisy, excited, or declamatory manner	talk in a noisy, excited, or declamatory manner
	gush forth in a sudden stream or jet