sheathe vs plunge

sheathe is not a noun while plunge is a noun.

Word	Noun	Adjective	Verb	Adverb
sheathe	No	No	Yes	No
plunge	Yes	No	Yes	No

As verbs, plunge is a hypernym of sheathe; that is, plunge is a word with a broader meaning than sheathe:

Other hypernyms of sheathe include immerse.

sheathe (noun)	plunge (noun)
	a brief swim in water
	a steep and rapid fall

sheathe (verb)	plunge (verb)
enclose with a sheath	begin with vigor
cover with a protective sheathing	devote (oneself) fully to
plunge or bury (a knife or sword) in flesh	cause to be immersed
	immerse briefly into a liquid so as to wet, coat, or saturate
	thrust or throw into
	fall abruptly
	drop steeply
	dash violently or with great speed or impetuosity