back up vs silt

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

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

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

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

back up (noun)	silt (noun)
	mud or clay or small rocks deposited by a river or lake

back up (verb)	silt (verb)
establish as valid or genuine	become chocked with silt
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