back up vs computer

back up is not a noun while computer is a noun.

Word	Noun	Adjective	Verb	Adverb
back up	No	No	Yes	No
computer	Yes	No	No	No

back up (noun)	computer (noun)
	a machine for performing calculations automatically
	an expert at calculation (or at operating calculating machines)

back up (verb)	computer (verb)
establish as valid or genuine
become or cause to become obstructed
make a copy of (a computer file) especially for storage in another place as a security copy
move backwards from a certain position
give moral or psychological support, aid, or courage to