paperknife vs paper knife

paperknife and paper knife both are nouns.

Word	Noun	Adjective	Verb	Adverb
paperknife	Yes	No	No	No
paper knife	Yes	No	No	No

As nouns, paperknife and paper knife are synonyms defined as:

paperknife and paper knife: dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books

Other synonyms of paperknife include letter opener.

paperknife (noun)	paper knife (noun)
dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books	dull knife used to cut open the envelopes in which letters are mailed or to slit uncut pages of books