Definitions:

1. to become or cause to become obstructed verb
   • The car crash made the highway back up.
2. to establish as valid or genuine verb
   • Can you back up your claims?
3. to give moral or psychological support, aid, or courage to verb
   • Go ahead and tell them how you feel; we'll back you up.
4. to make a copy of a computer file, especially for storage in another place as a security copy verb
   • You'd better back up these files!
5. to move backwards from a certain position verb
   • We should back up and discuss what happened before that.

Synonyms:

• back
• back down
• back off
• clog
• support