The Collaborative International Dictionary of English v.0.48:

Fortuitous \For*tu"i*tous\, a. [L. fortuitus; akin to forte,
   adv., by chance, prop. abl. of fors, fortis, chance. See
   Fortune.]
   1. Happening by chance; coming or occuring unexpectedly, or
      without any known cause; chance; as, the fortuitous
      concourse of atoms.
      [1913 Webster]

            It was from causes seemingly fortuitous . . . that
            all the mighty effects of the Reformation flowed.
                                                  --Robertson.
      [1913 Webster]

            So as to throw a glancing and fortuitous light upon
            the whole.                            --Hazlitt.
      [1913 Webster]

   2. (LAw) Happening independently of human will or means of
      foresight; resulting from unavoidable physical causes.
      --Abbott.

   Syn: Accidental; casual; contingent; incidental. See
        Accidental. -- For*tu"i*tous*ly, adv. --
        For*tu"i*tous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fortuitousness
    n 1: the quality of happening accidentally and by lucky chance