back up vs undergird

back up and undergird both are verbs.

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

As verbs, undergird is a hyponym of back up; that is, undergird is a word with a more specific, narrower meaning than back up:

back up (verb)	undergird (verb)
establish as valid or genuine	make secure underneath
become or cause to become obstructed	lend moral support to
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