facultyyoudaoicibaDictYouDict[faculty 词源字典]
faculty: [14] If one has a faculty for doing something, one finds it ‘easy’ to do. The word comes, via Old French faculte, from Latin facultās. This was a parallel form to facilitās (source of English facility [15]). Both were derived from Latin facilis ‘easy’ (whence English facile [15]), an adjective formed from the verb facere ‘do’. Since facilitās more closely resembled facilis, it retained its connotations of ‘easiness’, whereas by the classical period facultās had more or less lost them, coming to mean ‘capability, power’.
=> facile, facility[faculty etymology, faculty origin, 英语词源]
faculty (n.)youdaoicibaDictYouDict
late 14c., "ability, opportunity, means, resources," from Old French faculte "skill, accomplishment, learning" (14c., Modern French faculté) and directly from Latin facultatem (nominative facultas) "power, ability, capability, opportunity; sufficient number, abundance, wealth," from *facli-tat-s, from facilis (see facile).

Academic sense "branch of knowledge" (late 14c.) was in Old French and probably was the earliest in English (it is attested in Anglo-Latin from late 12c.), on notion of "ability in knowledge" or "body of persons on whom are conferred specific professional powers." Originally each department was a faculty; the use in reference to the whole teaching staff of an entire college dates from 1767. Related: Facultative. The Latin words facultas and facilitas "were originally different forms of the same word; the latter, owing to its more obvious relationship to the adj., retained the primary sense of 'easiness', which the former had ceased to have before the classical period." [OED]