back up vs congest

back up and congest both are verbs.

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

As verbs, back up and congest are synonyms defined as:

Other synonyms of back up include choke, choke off, clog, clog up, crap up, foul.

back up (verb)	congest (verb)
establish as valid or genuine	become or cause to become obstructed
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