think on one's feet 隨機應便；當機立斷

解釋：

CNN互動英語學習報 -六月二日: "on one's feet 就是指某人「站著」，而 think on one's feet 就是指某人可以一邊行動、一邊思考，也就是指其「反應靈敏」的意思。"

例句：

• My boss is looking for someone who can think on his feet to take over the Beijing office. 我老闆正在找一個反應靈敏的人來接管北京分公司。

• The ability to think on his feet. 他思想敏捷。