Source : Webster's Revised Unabridged Dictionary (1913)

Southdown \South"down`\, a.
   Of or pertaining to the South Downs, a range of pasture hills
   south of the Thames, in England.

   {Southdown sheep} (Zo["o]l.), a celebrated breed of
      shortwooled, hornless sheep, highly valued on account of
      the delicacy of their flesh. So called from the South
      Downs where the breed originated.