means A hard pointed growth on the head of certain animals, or any of various objects and instruments shaped like or made from one, including the brass tube you blow to make sound or the device on a car that blares a warning.

from From Old English 'horn,' a deeply ancient word traced back to Proto-Germanic '*hurnaz' and the Proto-Indo-European root '*ker-' meaning 'head' or 'horn' — the same root family that gives us Latin 'cornu' (whence 'cornet' and 'unicorn') and Greek 'keras.' So the bull's weapon, the trumpet, and the rhinoceros all share one prehistoric ancestor; humans simply blew through the hollow ones and kept the name.