I had always thought this word carried the connotation of surprise, but the definitions seem to belie that.
1. Of the nature of an addition from without; extrinsically added, not essentially inherent; supervenient, accidental, casual. Cf. the obs. ADVENTIVE.
2. Law. (See quot. 1751.) 1751 CHAMBERS Cycl., Adventitious, in the Civil Law, is applied to such goods as fall to a man, either by mere fortune, or by the liberality of a stranger, or by collateral, not direct succession. In this sense the word stands opposed to Profectitious; by which are signified such goods as descend in a direct line.
3. Nat. Hist. Appearing casually, or out of the normal or usual place, esp. in Bot. of roots, shoots, buds, etc. produced in unusual parts of the plant.