to move backwards, especially in a vehicle

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.

to provide support for somebody/something

two doctors backed up by a team of nurses

The rebels backed up their demands with threats.

to prepare a second copy of a file, program, etc. that can be used if the main one fails or needs extra support

It's a good idea to back up your files onto a memory stick.

LEARNit Dictionary