· play dumb with someone装聋作哑

It"s no use playing dumb with teachers. They know very well their students.
· be down in dumps垂头丧气，深情沮丧

I burned midnight oil yesterday. So I feel down in the dumps today.

· make the dust fly干得很起劲

Many volunteers are helping in the Olympic Games. They are making the dust fly without getting pay.

· go easy on...酌情处理……；节约使用……；对……手下留情

I hope the teacher will go easy on me. Otherwise, I"ll have no chance to pass the exam.

当介词on的后面跟得是something的时候，此语用来是“节约使用”或“小心对待”
Go easy on salt. We have to save some for the next meal.

· egg someone on to do something怂恿，鼓动某人干某事

Haven"t you seen him through? He can help you nothing but egg you on to do illegal things.

· rub elbows with someone和某人没有很深的交情

I suppose Jack doesn"t know me well. We have only rubbed elbows with other for several times.

· be at the end of one"s tether山穷水尽

Mark feels worried because he can"t write any new books. He is afraid that he is at the end of his tether.

· be at one"s wits" end不知如何是好

My boy is such a mischief-maker. I am at my wits" end with him.

· be at loose ends心里没有着落

Many old people feel at loose ends after their retirement.

· make both ends meet收支平衡

The newly-wed couple found it hard to make both ends meet.

· be green with envy羡慕极了

Don"t be green with envy. He deserves the prize, since he has spent a whole month on that paper.

· be of the essence绝对不可少，关键的

As for study, diligence is of the essence.

· a poor apology for...蹩脚货，名不副实

That travel agency was first-class, but it offered a poor apology for its service

· make an exhibition of oneself出洋相

如果表示总出洋相可以在exhibition一词前加上形容词， regular
She is a bit insane. She always makes a regular exhibition of herself by raging at almost everybody.

· keep an eye open密切注视

Keep an eye open for anyone who arouses your suspicion.