the.com/door knocker

means A hinged metal fixture, usually mounted on a front door, that you rap against a plate to announce yourself before anyone invents the doorbell.

from Door knockers trace back to simple metal rings and hammers used across ancient and medieval doors — the Greeks and Romans had versions, and grand knockers (lion heads, hands, gargoyle faces) became a status symbol in European homes by the 1600s–1700s, since having one at all meant you had a door worth guarding and visitors worth announcing. The word itself is just literal: 'knock' plus 'er,' the thing that does the knocking — no hidden trick there, the ornament did all the fancy work, the name stayed plain.