undergird vs back up

undergird and back up both are verbs.

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

As verbs, back up is a hypernym of undergird; that is, back up is a word with a broader meaning than undergird:

Other hypernyms of undergird include support.

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