back up phrasal verb ( see also ↑ back )
1 . back somebody/something ↔ up to say or show that what someone is saying is true:
Jane would back me up if she were here.
There’s no evidence to back up his accusations.
These theories have not been backed up by research.
2 . back somebody/something ↔ up to provide support or help for someone or something:
The plan’s success depends on how vigorously the UN will back it up with action.
The police officers are backed up by extra teams of people at the weekend.
⇨ ↑ backup
3 . to make a copy of information stored on a computer:
Make sure you back up.
back something ↔ up
These devices can back up the whole system.
back something ↔ up onto something
Back all your files up onto floppy disks.
⇨ ↑ backup
4 . especially American English to make a vehicle move backwards:
The truck stopped and then backed up.
back something ↔ up
I backed the car up a little.
5 . to move backwards:
Back up a bit so that everyone can see.
⇨ ↑ backup
6 . if traffic backs up, it forms a long line of vehicles that cannot move:
The traffic was starting to back up in both directions.
7 . if a toilet, sink etc backs up, it becomes blocked so that water cannot flow out of it