She admitted sheepishly that she had forgotten her friend's birthday.

sheepishly 是由名词 sheep（绵羊）加上形容词后缀 -ish（具有...特性的）构成形容词 sheepish（羞怯的、局促不安的），再加上副词后缀 -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、不好意思地"这种状态。