Fortuitous, a.
Etymology: [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.
Shades of meaning
Fernald lists fortuitous only as an antonym, under inherent.
He lists it without comment; the entries above explain its neighbors.