He got his hair so short it looked like it was chopped off.

chopped off 是动词短语 chop off 的过去分词形式。chop 意思是"砍、劈"，off 表示"离开、断开"。chopped off 字面意思是"被砍断"，在这里形容头发被剪得特别短，像是被砍掉一样。