《论衡·卷十二·量知篇》
素就是空，空虚没有道德，又拿别人的俸禄，所以叫做白吃饭。
素者，空也；空虚无德，餐人之禄，故曰素餐。
The word "su" means empty. They were empty of virtues and received salaries from others, so they were called those who received salaries without working.