The best man has a hollow leg. He drinks a lot instead of the bridegroom.
* pull someone's leg开某人的玩笑
Never ever pull his leg. He is a man without a sense of humor. He takes everything seriously.
* keep someone at arm's length与某人保持距离
My boss is my uncle. In order to avoid unnecessary misunderstanding, I'd better keep him at arm's length in company.
* let someone down使某人失望
Joe really let me down by having broken his promises for serveral times.
* can't for the life of one怎么……也不行
I thought I could recite my speech backwards, but when I stood in front of the audience, I couldn't for the life of me remember what to say.