1.drop somebody a line = contact somebody 通常指写信，留纸条或者电话联系某人
I know you'll be busy enjoying yourself, but please, drop me a line on your trip.
我知道你到时候肯定忙着玩儿，但是，一定，旅途中要给我个信。
I'll drop you a line once I get settled in at school.
我在学校安顿好了就联系你们。
2.drop 掉落
I dropped my glasses and broke them.
我不小心把眼镜弄掉了，然后摔碎了。
3.at the drop of a hat a. 指一个脾气一触即发的人/ b. 毫不迟疑地，一有机会就立即行动
Say something she doesn't agree with, and she'll start aloud argument at the drop of a hat.
说点啥她不同意的话，她就要和你好好大声争论一番了。
She'd buy herself expensive clothing at the drop of a hat and worry about how she would pay for it later.
她眼睛都不眨毫不犹豫地给自己买下了昂贵的衣服，然后开始担心她要怎么样还清账单。
If you need help, just call on me. I can come at the drop of a hat.
如果你需要帮助，打电话给我。我会毫不犹豫地前来帮你。
4.a drop of water 一滴水
a drop of tear 一滴眼泪
a drop of blood 一滴血
5.a drop in the bucket/ocean 沧海一粟，九牛一毛，十分微小
What I have done is just a drop in the bucket.
我所做的只不过是杯水车薪，微不足道的一些事情。
