To back down is to admit defeat.
To back off is to draw back from opposing someone.
To back out is to withdraw from something you have promised to do.
- He had promised to lend me a loan but he backed out in the end.
Back something up
- You must always back up your data.
Behind someone’s back
When you do something behind someone’s back, you do it without their knowledge.
- They were saying nasty things about me behind my back.
Get someone’s back up / Put someone’s back up
To get someone’s back up is to annoy them.
- I got his back up with my stupid remarks.
Turn your back on
To turn your back on someone is to reject or ignore them.
- When at last he returned to her, she turned her back on him.
Have your back to the wall / Have your back up against the wall
When you have your back to the wall, you are in a very difficult situation.
- He has borrowed money from everyone – he really has his back to the wall now.