From Longman Dictionary of Contemporary Englishon the ballon the ballinformalINTELLIGENT able to think or act quickly and intelligently an assistant who’s really on the ball → ball
Examples from the Corpus
on the ball• The cricketer himself seems intent on something earlier; back on the ball by June.• The thinking behind that extra half inch was it would give me better plate coverage on the ball away.• Parks is a man who knows how to keep his eye on the ball.• Calluses grow on the ball of the foot and do not have a nucleus.• They were the black men who did, indeed, have it on the ball.• Super slow-motion replays appeared to show O'Driscoll did not apply downward pressure on the ball.• There seemed to be no time on the ball, and poor Dylan Kerr was run ragged down the right wing.• He was turning on the ball right away against veteran pitchers.• A photographer who was on the ball got some great pictures of the incident. 