I recently attending the 500th anniversary dinner of my school, based in the English Midlands, which was attended by hundreds of well-educated old boys, including the Governor of the Bank of England, who spoke eloquently and well. He was followed in the speeches by the current headmaster (in what would surely be the most high-profile speech of his life, doubtless hoping all the successful alumni present would dig deep into their pockets to help the school) and on about four occasions he referred to this person or that person as "sat at the back". Each time he said it I winced internally, and all I could think was how much standards at the school must have dropped. It may be an idiom, but soem peopel need to learn the correct moment to use it. This was not one of them!