Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

            So as to throw a glancing and fortuitous light upon
            the whole.                            --Hazlitt.

   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.

Source : WordNet®

fortuitous
     adj 1: having no cause or apparent cause; "a causeless miracle";
            "fortuitous encounters--strange accidents of fortune";
            "we cannot regard artistic invention as...uncaused and
            unrelated to the times" [syn: {causeless}, {uncaused}]
     2: occurring by happy chance; "profits were enhanced by a
        fortuitous drop in the cost of raw materials"