spout vs mouth off

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

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

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

Other synonyms of spout include jabber, rabbit on, rant, rave.

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

spout (verb)	mouth off (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