a household word
Pearl S. Buck was almost a household word throughout much of her lifetime.
＝A widely known saying, name, person, or thing

我想知道为什么我回的帖子看不见！！

a needle in a haystack
We are supposed to meet John here at the train station.
—That's like looking for a needle in a haystack（干草堆）.
＝something impossible to do（大海捞针）

a nest egg
Today the simple piggy bank is seen everywhere as the symbol of saving and frugality, for building a nest egg for life's sudden money needs.
＝A sum of money put by as a reserve 应急款

a rainy day
We should not spend all our money. Instead, we should put away some for a rainy day.
＝A time of need or trouble

a beach person  
啥意思 ？

a rule of thumb
This basic chemical knowledge was applied in most cases as a rule of thumb.
＝A useful principle having wide application but not intended to be strictly accurate or reliable in every situation（土办法）

a scorcher
Whew! It's a real scorcher today.
＝a hot day

a beach person
我也想问一下，难道只是直译？

i think that's pretty exciting