bread and butter：生活，生计

e.g. It's bread and butter to me. I have to do it.

这是我的生计，我得去做。

e.g. I can't give it up. It's my bread and butter.

我不能放弃它，我全靠它生活了。

相关词：

bread and butter letter：（拜访、见面或谈话以后发出的）有礼貌的信函

e.g. I sat down after I got home and wrote a stack of bread and butter letters.

到家后，我坐下来写了一大堆致谢信。

e.g. I got a bread and butter letter from Frank. I should have sent him one first.

我收到弗兰克寄来的致谢信，其实我应该先给他写信才是。