don't let the grass grow under feet别理解成“不让草长在脚下”，其正确的含义是保持积极主动，（做事）不要拖拉，不磨洋工，及时行动。

【例句】
Don't let the grass grow under your feet, kid. Get to work!
请保持积极主动，孩子。赶紧去工作吧！

Get your job done quickly. Don't let the grass grow under your feet.
赶快做好你的工作，别再拖拖拉拉了。