Alert, competent, or efficient:警惕的，有能力的或有效率的：a teacher who is really on the ball.能干的老师
Relating to qualities, such as competence, skill, or knowledge, that are necessary for success:成功的必备条件：关于能力、技巧或知识等成功所必要的品质：a manager who has a lot on the ball; a student who has nothing on the ball.一个很能干的经理；一名什么也不会的学生