He admitted sheepishly that he had forgotten to bring his homework.

sheepishly 是由名词 sheep（绵羊）加上后缀 -ish（具有...特性的）和副词后缀 -ly 构成的副词。绵羊常被认为性格温顺害羞，因此 sheepishly 形容"像绵羊般羞怯地"或"不好意思地"。