back up vs lug

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

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

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

Other hyponyms of back up include gum up, block, choke up, stuff, silt, silt up.

back up (noun)	lug (noun)
	marine worms having a row of tufted gills along each side of the back; often used for fishing bait
	a projecting piece that is used to lift or support or turn something
	a sail with four corners that is hoisted from a yard that is oblique to the mast

back up (verb)	lug (verb)
establish as valid or genuine	carry with difficulty
become or cause to become obstructed	obstruct
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