Source : Webster's Revised Unabridged Dictionary (1913)

Sunk \Sunk\,
   imp. & p. p. of {Sink}.

   {Sunk fence}, a ditch with a retaining wall, used to divide
      lands without defacing a landscape; a ha-ha.

Source : WordNet®

sunk fence
     n : a ditch with one side being a retaining wall; used to divide
         lands without defacing the landscape [syn: {ha-ha}, {haw-haw}]