1563, from M.Fr. portente, from L. portentum "a sign, omen," from neut. of portentus, pp. of portendre (see portend). Portentous is recorded from c.1540.
[Hey, BD, "portendre" is missing a semipenultimate vowel, isn't it? ...I thought I just made up "semipenultimate", but I see others have been using it, too. I hope they've been using it right! ]
-TimOmens or portents are signs encountered fortuitously that are believed to foretell the future. Their interpretation is a form of divination.