She advised him to think twice about quitting his stable job.

think 的意思是"思考、考虑"。 twice 是副词，意思是"两次"，由 two（二）加后缀 -ice 构成，表示"两次"或"两倍"。 think twice 是一个固定短语，字面意思是"思考两次"，引申为"三思而后行"或"慎重考虑"。