The Collaborative International Dictionary of English v.0.48:

-ous \-ous\suff. [OF. -ous, us, -os, F. -eux, fr. L. -osus, and
   -us. Cf. -ose.]
   1. An adjective suffix meaning full of, abounding in, having,
      possessing the qualities of, like; as in gracious,
      abounding in grace; arduous, full of ardor; bulbous,
      having bulbs, bulblike; riotous, poisonous, piteous,
      joyous, etc.
      [1913 Webster]

   2. (Chem.) A suffix denoting that the element indicated by
      the name bearing it, has a valence lower than that denoted
      by the termination -ic; as, nitrous, mercurous, ferrous,
      sulphurous, etc., as contrasted with nitric, mercuric,
      ferric, sulphuric, etc.
      [1913 Webster]