Source : Webster's Revised Unabridged Dictionary (1913)

Pet \Pet\, a.
   Petted; indulged; admired; cherished; as, a pet child; a pet
   lamb; a pet theory.

         Some young lady's pet curate.            --F. Harrison.

   {Pet cock}. [Perh. for petty cock.] (Mach.) A little faucet
      in a water pipe or pump, to let air out, or at the end of
      a steam cylinder, to drain it.