Very interesting, thanks. http://atilf.atilf.fr and Dictionnaire historique de la langue Française both see two meanings for the French "trivial": one is common as in commonplace, and the other is common as in gross and vulgar. The first comes from /trivialis/, as fhars says. The latter comes from /trivium/, the crossroads. I don't know if English also has the second meaning: vulgar.