to support or help someone

My family backed me up throughout the court case. 打官司期间，我的家人一直在支持我。

to say that someone is telling the truth

Honestly, that's exactly what happened - Claire will back me up. 实话实说，这就是当时的情况——克莱尔会为我作证。

Will you back me up if I say that I never saw him? 如果我说从未见过他，你会为我作证吗？

to prove something is true

His claims are backed up by recent research. 最新的研究证实了他的断言。

to make an extra copy of computer information

Make sure you back up your files. 确保你的文件备份了。

See also: backup noun

to drive backwards

If traffic backs up, the vehicles have to wait in a long line because there are too many of them.

The traffic is starting to back up on the M25. 25号高速公路开始堵车了。