I met her the next day and tried to make an excuse to back out of the deal, but she didn't let me.

back 在这里是动词，本意是"后退、倒退"，引申为"撤回、取消"的意思。 out 是副词，表示"向外、离开"的状态。 back out 是一个动词短语，意思是"退出、食言"，特指从已经承诺的事情中反悔或撤出。