Get something done in one vigorous effort

During the Spring and Autumn Period, an army from the State of Qi confronted one from the State of Lu. After the first roll of drums from the Qi side to summon Lu to battle, the Lu ruler wanted to attack. But his counsellor Cao Gui said, 'We should wait until the third drum roll, sire.' After the Qi side had beaten the drums three times, the Lu army attacked and defeated the Qi army. After the battle, the king asked Cao Gui the reason for his odd advice. Cao Gui answered, 'Fighting needs spirit. Their spirit was aroused by the first roll or the drums, but was depleted by the second. And it was completely exhausted by the third. We started to attack when their spirit was exhausted. That's why we won.' 

春秋时代，齐国派兵攻打鲁国。鲁国的国王鲁庄公带着谋士曹刿指挥作战。齐军第一次击鼓以后，鲁军准备发起进攻。曹刿说：“不行。”齐军三次击鼓以后，曹刿才说：“现在可以进攻了。”结果齐军大败。战斗结束后，鲁庄公问曹刿胜利的原因。曹刿说：“打仗要考勇气。第一次击鼓，士气十分旺盛；第二次击鼓，士气有些衰落；第三次击鼓，士气就消耗尽了。敌人士气耗尽，我们发起进攻，所以取得了胜利。”

This idiom later meant to get something done with one sustained effort.

后来，“一鼓作气”形容鼓起劲头，一下子把事情干完。