"of the nature of an addition from without," c.1600, from Medieval Latin adventitius "coming from abroad, extraneous," a corruption of Latin adventicius "foreign, strange, accidental," from advent- past participle stem of advenire "arrive" (see advent). Related: Adventitiously; adventitiousness.
adventitious ad·ven·ti·tious (ād'věn-tĭsh'əs, -vən-)
Arising from an external source or occurring in an unusual place or manner; extrinsic.
Occurring accidentally or spontaneously, not caused by heredity.