The doctor asked him to move his arm laterally to check for side pain.

laterally 是形容词 lateral（侧面的）加上副词后缀 -ly 构成的副词形式。lateral 来自拉丁语 lateralis，表示"侧面的、旁边的"。laterally 表示"向侧面地、横向地"，描述动作的方向。