English Trainer Chronicles: Grass root

A few minutes before this, my student told me, with a laugh, that he had asked his secretary for help with his English homework.

We were talking about idioms related to money.
Grass roots. The term "grass roots" refers to the ordinary people who form the main body of an organization.
Him: For example, my secretary is a grass root.
Me: Oh, noooo... when you say it that way, the meaning becomes literal.
Him: Oh. (Laughs out loud.) I will not be mentioning this.