The Collaborative International Dictionary of English v.0.48:

Plowshare \Plow"share`\, Ploughshare \Plough"share"\, n.
   The share of a plow, or that part which cuts the slice of
   earth or sod at the bottom of the furrow.
   [1913 Webster]

   Plowshare bone (Anat.), the pygostyle.
      [1913 Webster] Plowtail