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.
      [1913 Webster]

   4. (Med.) Acquired, as diseases; not hereditary. --AS
      [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

adventitious
    adj 1: associated by chance and not an integral part; "poetry is
           something to which words are the accidental, not by any
           means the essential form"- Frederick W. Robertson; "they
           had to decide whether his misconduct was adventitious or
           the result of a flaw in his character"

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "adventitious":
   accessory, accidental, additional, adscititious, aleatory,
   appurtenant, ascititious, auxiliary, casual, causeless, chance,
   chancy, circumstantial, collateral, conditional, contingent,
   destinal, dicey, extra, fatal, fatidic, fluky, fortuitous, iffy,
   incidental, indeterminate, inessential, nonessential, occasional,
   provisional, risky, secondary, subsidiary, superadded, superfluous,
   supervenient, supplemental, supplementary, uncaused, undetermined,
   unessential, unexpected, unforeseeable, unforeseen, unlooked-for,
   unpredictable



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ADVENTITIOUS, adventitius. From advenio; what comes incidentally; us
adventitia bona, goods that, fall to a man otherwise than by inheritance; or
adventitia dos, a dowry or portion given by some other friend beside the
parent.