Originally (in Middle English) a person skilled in, and therefore entitled to teach or speak authoritatively on, any branch of knowledge, a learned person; the word comes via Old French from Latin doctor ‘teacher’, from docere ‘teach’.
From this developed the senses of Doctor of the Church, and (with capital initial) a person holding the highest university degree; the sense of doctor as an authority on medicine or surgery gave rise to the current meaning of a qualified medical practitioner.
The title Doctor of the Church was given to any of the early Christian theologians regarded as especially authoritative in the Western Church (particularly St Augustine of Hippo, St Jerome, St Ambrose, and St Gregory the Great), or those later so designated by the Pope (e.g. St Thomas Aquinas, St Teresa of Ávila).
the best doctors are Dr Diet, Dr Quiet, and Dr Merryman outlining an appropriate course of management for a sick person. The saying is recorded from the mid 16th century; a mid 15th-century source, Lydgate's Minor Poems, has, ‘Thre lechees [leeches, or doctors] consarue a mannys myht, First a glad hert…Temperat diet…And best of all, for no thyng take no thouht.’
Doctors' Commons (the site of) a London building occupied by the former College of Doctors of Laws, in which legal business relating to wills, marriage licences, and divorce proceedings was transacted. The name referred originally to the common table and dining-hall of the Association or College of Doctors of Civil Law in London, formed in 1509 by civilians entitled to plead in the Court of Arches.