Back up has 6 different meanings across 1 category:
Verb
give moral or psychological support, aid, or courage to
"She supported him during the illness"
"Her children always backed her up"
move backwards from a certain position
"The bully had to back down"
establish as valid or genuine
"Can you back up your claims?"
make a copy of (a computer file) especially for storage in another place as a security copy
"You'd better back up these files!"
become or cause to become obstructed
"The leaves clog our drains in the Fall"
"The water pipe is backed up"
To move backwards, especially for a vehicle to do so.