A knocker is a piece of metal on the front door of a building, which you use to hit the door in order to attract the attention of the people inside. 门环
noun [专属名词]门环;敲击者;吹毛求疵的人
I felt loath to sully the gleaming brass knocker by handling it. 我不愿意用手去抓那个闪亮的黄铜门环以免把它弄脏。