汗牛充栋hàn niú chōng dòng形容书籍极多{enough books to make a pack-ox sweat or to fill a house to the rafters — an immense collection of books}(汗牛{hanniu}: 用牛运输,牛累得出汗{make the ox carrying the books perspire}; 充栋{chongdong}: 堆满了屋子{fill a house to the rafters})。