green是绿色的，as green as grass除了说像草一样绿，引申为少不更事，无生活经验的。

【例句】
When she left school and started her first job, she was as green as grass.
她走出校门刚开始工作时少不更事。

This is a very important position, so I can't understand why they hired someone who's as green as grass.
这是一个非常重要的职位，所以我不能理解为什么他们雇佣没有经验的人。