The Collaborative International Dictionary of English v.0.48:

Risible \Ris"i*ble\, a. [F., fr. L. risibilis, fr. ridere,
   risum, to laugh. Cf. Ridiculous.]
   1. Having the faculty or power of laughing; disposed to
      laugh.
      [1913 Webster]

            Laughing is our busines, . . . it has been made the
            definition of man that he is risible. --Dr. H. More.
      [1913 Webster]

   2. Exciting laughter; worthy to be laughed at; amusing.
      "Risible absurdities." --Johnson.
      [1913 Webster]

            I hope you find nothing risible in my complaisance.
                                                  --Sir W.
                                                  Scott.
      [1913 Webster]

   3. Used in, or expressing, laughter; as, risible muscles.
      [1913 Webster]

   Note: Risible is sometimes used as a noun, in the plural, for
         the feeling of amusement and for the muscles and other
         organs used in laughing, collectively; as, unable to
         control one's risibles.
         [1913 Webster]

   Syn: Ludicrous; laughable; amusing; ridiculous -- Risible,
        Ludicrous, Ridiculous.

   Usage: Risible differs from ludicrous as species from genus;
          ludicrous expressing that which is playful and
          sportive; risible, that which may excite laughter.
          Risible differs from ridiculous, as the latter implies
          something contemptuous, and risible does not.
          [1913 Webster] --Ris"i*ble*ness, n. -- Ris"i*bly,
          adv.
          [1913 Webster]