How early was this term used?
The term cardinal at one time applied to any priest permanently assigned or incardinated to a church, or specifically to the senior priest of an important church, based on the Latin cardo (hinge), meaning "principal" or "chief". The term was applied in this sense as early as the ninth century to the priests of the tituli (parishes) of the diocese of Rome. The Church of England retains an instance of this origin of the title, which is held by the two senior members of the College of Minor Canons of St Paul's Cathedral.