to move backwards, especially in a vehicle You can back up another two feet or so. I backed the car up to the door.
- 1 to support somebody/something; to say that what somebody says, etc. is true I'll back you up if they don't believe you. The writer doesn't back up his opinions with examples.
- 2 to provide support for somebody/something two doctors backed up by a team of nurses The rebels backed up their demands with threats. related noun backup