on the fly

副词 on the fly:

1. on the run or in a hurry

1. The bird caught a bug on the fly.
   那鸟在飞行中捉住了一只昆虫。
2. John is very busy, and people who want to talk to him have to catch him on the fly.
   约翰非常忙，人们要找他谈话只能在他百忙中瞅个空子。
3. Housework keeps mother on the fly all day.
   家务劳动使母亲整天忙忙碌碌。