petcock

from WordNet (r) 3.0 (2006)

petcock
    n 1: regulator consisting of a small cock or faucet or valve for
         letting out air or releasing compression or draining

from 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]

from The Collaborative International Dictionary of English v.0.48

petcock \pet"cock`\ (p[e^]t"k[o^]k`) n.
   A small cock or faucet or valve for letting out air or
   releasing compression, or for draining a vessel.
   [WordNet 1.5]