After signing the contract, she had second thoughts and decided to back out.

back 在这里是动词，意思是"后退、撤回"。 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、撤回承诺"，通常指在做出承诺或签订协议后改变主意。