at the drop of a hat：1、立即，马上

e.g. If you need a babysiter quickly, call Mary, because she can come at the drop of a hat.

要是你急需保姆就去叫玛丽，她一叫就到。

2、动不动（就）

e.g. He was quarrelsome and ready to fight at the drop of a hat.

他好争吵，动不动就摆出一副与人争斗的样子。

e.g. He loses his temper at the drop of a hat.

他动不动就发脾气。