The Collaborative International Dictionary of English v.0.48:

adventitious \ad`ven*ti"tious\ ([a^]d`v[e^]n*t[i^]sh"[u^]s), a.
   [L. adventitius.]
   1. Added extrinsically; not essentially inherent.

   Syn: accidental; casual; incidental; additional;
        supervenient; foreign.
        [1913 Webster + PJC]

              To things of great dimensions, if we annex an
              adventitious idea of terror, they become without
              comparison greater.                 --Burke.
        [1913 Webster]

   2. (Nat. Hist.) Out of the proper or usual place; as,
      adventitious buds or roots.
      [1913 Webster]

   3. (Bot.) Accidentally or sparingly spontaneous in a country
      or district; not fully naturalized; adventive; -- applied
      to foreign plants.
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   4. (Med.) Acquired, as diseases; not hereditary. --AS
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   5. (Med.) Occurring in an unusual place or arising from an
      external source. --Stedman 25
      [PC] -- Ad`ven*ti"tious*ly, adv. --
      Ad`ven*ti"tious*ness, n.
      [1913 Webster]