His hair looks wild and long because it's been left unclipped for months.

unclipped 是由前缀 un-（表示"不、未"）和动词 clip（剪）的过去分词 clipped 组合而成，字面意思是"未被修剪的"。unclipped 在这里作形容词，描述头发"长时间未经修剪"的状态。