The Collaborative International Dictionary of English v.0.48:

pet \pet\ (p[e^]t), n. [Formerly peat, perhaps from Ir. peat,
   akin to Gael. peata.]
   1. A cade lamb; a lamb brought up by hand.
      [1913 Webster]

   2. Any person especially cherished and indulged; a fondling;
      a darling; often, a favorite child; as, a teacher's pet.
      [1913 Webster + PJC]

            The love of cronies, pets, and favorites. --Tatler.
      [1913 Webster]

   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.
      [1913 Webster]

   4. Any animal kept as a companion, usually in or around one's
      home, typically domesticated and cared for attentively and
      often affectionately. Distinguished from animals raised
      for food or to perform useful tasks, as a draft animal
      or a farm animal.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

pet \pet\, a.
   Petted; indulged; favorite; admired; cherished; as, a pet
   child; a pet lamb; a pet theory; a pet animal.
   [1913 Webster]

         Some young lady's pet curate.            --F. Harrison.
   [1913 Webster]

   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. [Also spelled petcock.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pet \Pet\, v. i.
   To be a pet. --Feltham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pet
    adj 1: preferred above all others and treated with partiality;
           "the favored child" [syn: favored, favorite(a),
           favourite(a), best-loved, pet, preferred,
           preferent]
    n 1: a domesticated animal kept for companionship or amusement
    2: a special loved one [syn: darling, favorite, favourite,
       pet, 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, PET]
    v 1: stroke or caress gently; "pet the lamb"
    2: stroke or caress in an erotic manner, as during lovemaking