1. withdraw from something or decide not to fulfill a promise
back out of something at the last minute
When you 'back out' of a course of action or an ongoing task or project, you leave it or decide not to pursue it any further. To 'back out' can mean to reverse a vehicle or to reverse a decision. This is usually used when the person 'backing out' did so unexpectedly, an action resulting in other people being let down or disappointed.