case knife vs sheath knife

case knife and sheath knife both are nouns.

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

As nouns, case knife and sheath knife are synonyms defined as:

case knife and sheath knife: a knife with a fixed blade that is carried in a sheath

case knife (noun)	sheath knife (noun)
a knife with a fixed blade that is carried in a sheath	a knife with a fixed blade that is carried in a sheath
a metal blade with a handle; used as cutlery