doctor (n.) Look up doctor at Dictionary.com
c.1300, "Church father," from Old French doctour, from Medieval Latin doctor "religious teacher, adviser, scholar," in classical Latin "teacher," agent noun from docere "to show, teach, cause to know," originally "make to appear right," causative of decere "be seemly, fitting" (see decent).

Meaning "holder of highest degree in university" is first found late 14c.; as is that of "medical professional" (replacing native leech (n.2)), though this was not common till late 16c. The transitional stage is exemplified in Chaucer's Doctor of phesike (Latin physica came to be used extensively in Medieval Latin for medicina). Similar usage of the equivalent of doctor is colloquial in most European languages: Italian dottore, French docteur, German doktor, Lithuanian daktaras, though these are typically not the main word in those languages for a medical healer. For similar evolution, see Sanskrit vaidya- "medical doctor," literally "one versed in science." German Arzt, Dutch arts are from Late Latin archiater, from Greek arkhiatros "chief healer," hence "court physician." French médecin is a back-formation from médicine, replacing Old French miege, from Latin medicus.
doctor (v.) Look up doctor at Dictionary.com
1590s, "to confer a degree on," from doctor (n.). Meaning "to treat medically" is from 1712; sense of "alter, disguise, falsify" is from 1774. Related: Doctored; doctoring.