once in a lifetime

• 生命中的唯一一次；一生一次

once in a lifetime的用法和样例：

例句

1. Female sheep produce one or two lambs at a time.
   母羊一次生一到两只羊羔。
2. The hen gave him a gold egg every day.
   这只母鸡每天为他生一个金蛋。