She glanced nervously over her shoulder to make sure no one else was listening.

shoulder 是名词，指"肩膀"这个身体部位。在句中指"回头看的动作方向"。 glanced over one's shoulder 是一个固定搭配，表示"回头张望"的动作，字面意思是"视线越过肩膀看"。