sheath knife vs knife

sheath knife and knife both are nouns.

Word	Noun	Adjective	Verb	Adverb
sheath knife	Yes	No	No	No
knife	Yes	No	Yes	No

As nouns, knife is a hypernym of sheath knife; that is, knife is a word with a broader meaning than sheath knife:

sheath knife: a knife with a fixed blade that is carried in a sheath
knife: edge tool used as a cutting instrument; has a pointed blade with a sharp edge and a handle

sheath knife (noun)	knife (noun)
a knife with a fixed blade that is carried in a sheath	edge tool used as a cutting instrument; has a pointed blade with a sharp edge and a handle
	a weapon with a handle and blade with a sharp point
	any long thin projection that is transient

sheath knife (verb)	knife (verb)
	use a knife on