Cheng Yi (chéng yí 程颐) was a Chinese philosopher born in Luoyang during the Song Dynasty. He worked with his older brother Cheng Hao (chéng hào 程灏). On a heavy snowy day, there was a young man named Yangshi went to visit his teacher Cheng Yi. He found his teacher was sleeping when he arrived at the house. So he waited in the snow because he didn't want to wake his teacher up. When his teacher finally got up and the snow was very thick. Since then people use this idiom to say some one respect his teacher.