Thanks Mr.Pedantic,Thank all of you who have contributed in this thread.Frankly speaking,I still can't fully understand the comprehensive meaning of the phrase,but your explanation helps.I believe that the reason why I can't understand it wholely is that I still need to learn more about English culture,that way I think the more I learn ,the more I know.
Do you know some website refer to English idioms or slangs?I am looking forward to hearing from you.; -) or