Source : Webster's Revised Unabridged Dictionary (1913)

Pet \Pet\, n. [Formerly peat, perhaps from Ir. peat, akin to
   Gael. peata.]
   1. A cade lamb; a lamb brought up by hand.

   2. Any person or animal especially cherished and indulged; a
      fondling; a darling; often, a favorite child.

            The love of cronies, pets, and favorites. --Tatler.

   3. [Prob. fr. {Pet} a fondling, hence, the behavior or humor
      of a spoiled child.] A slight fit of peevishness or
      fretfulness. ``In a pet she started up.'' --Tennyson.

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.

Pet \Pet\, v. t. [imp. & p. p. {Petted}; p. pr. & vb. n.
   {Petting}.]
   To treat as a pet; to fondle; to indulge; as, she was petted
   and spoiled.

Pet \Pet\, v. i.
   To be a pet. --Feltham.

Source : WordNet®

pet
     adj : preferred above all others and treated with partiality; "the
           favored child" [syn: {favored}, {favorite(a)}, {favourite(a)},
            {preferred}]
     [also: {petting}, {petted}]

pet
     n 1: a domesticated animal kept for companionship or amusement
     2: a special loved one [syn: {darling}, {favorite}, {favourite},
         {dearie}, {deary}, {ducky}]
     3: a fit of petulance or sulkiness (especially at what is felt
        to be a slight)
     4: using a computerized radiographic technique to examine the
        metabolic activity in various tissues (especially in the
        brain) [syn: {positron emission tomography}]
     [also: {petting}, {petted}]

pet
     v 1: stroke or caress gently; "pet the lamb"
     2: stroke or caress in an erotic manner, as during lovemaking
     [also: {petting}, {petted}]