niú jiǎo guà shū
比喻读书勤奋。
《新唐书·李密传》:“闻包恺在缑山,往从之。以蒲鞯乘牛,挂《汉书》一帙角上,行且读。”
牛角挂书作定语;指勤奋读书。
read while grazing the cattle with one's books hung on its horns