Source : Webster's Revised Unabridged Dictionary (1913)

   2. A thing not discovered; what is unknown or unexplained; a
      mystery.

            All secrets of the deep, all nature's works.
                                                  --Milton

   3. pl. The parts which modesty and propriety require to be
      concealed; the genital organs.

   {In secret}, in a private place; in privacy or secrecy; in a
      state or place not seen; privately.

            Bread eaten in secret is pleasant.    --Prov. ix.
                                                  17.

Source : WordNet®

in secret
     adv : in secrecy; not openly; "met secretly to discuss the
           invasion plans"; "the children secretly went to the
           movies when they were supposed to be at the library";
           "they arranged to meet in secret" [syn: {secretly}, {on
           the Q.T.}, {on the QT}]