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 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

And how
The concert pianist was fantastic.
—And how!
＝当然罗!

as a rule
I don’t care much for horror movies as a rule.
＝In general; for the most part

as luck would have it
—As luck would have it, there was no one home. 不巧的是，没人在家
—As luck would have it, he recovered his lost watch. 幸运得是，他找回了他丢失的表。
＝幸运的是，不幸的是，碰巧

as ... go
This butterfly is tough and powerful as butterflies go and is capable of long flights at speeds of 20 miles per hour or more. 就蝴蝶而言，这种蝴蝶强劲有力，能够以时速20英里以上的速度长途飞行
＝就...而言

back up
The accident backed the traffic up for blocks. 事故导致交通堵塞了几条街
Traffic backed up in the tunnel. 交通在隧道里堵塞起来
＝To cause to accumulate or undergo accumulation 使堆积，堵塞

be … short
I found the perfect book bag, but I am about 20 dollars short. 我找到了理想的书包，可我钱不够，差20块
＝差。。。钱

be allergic to
Dan is allergic to cats. 丹对猫过敏
＝Having a dislike; averse 对。。。过敏

be beat
It's almost midnight. Why don't we leave those dishes until tomorrow?
—Good idea! I'm beat.
＝be very tired (筋疲力尽)