decorum

Decorum is a learned borrowing from Latin decōrum, the neuter form of the adjective decōrus, meaning "proper" or "decent."

The facts available don't show a shift in meaning — English simply took the Latin word directly, keeping its sense of fitting, proper behavior.

It shares its root with decōrus, the Latin word for "proper" or "decent," from which decorum's own sense of propriety comes.