He assumed an indifferent look as he walked into the room.

assumed 是动词 assume 的过去式。assume 由前缀 as-（表示"朝向"）和词根 sume（来自拉丁语 sumere，意为"拿取"）组成，本义是"采取、承担"。在这里引申为"假装、表现出某种样子"的意思。