rule of coss

from The Collaborative International Dictionary of English v.0.48

Coss \Coss\, n. [It. cosa.]
   A thing (only in phrase below).
   [1913 Webster]

   {Rule of Coss}, an old name for Algebra. [It. regola di cosa
      rule of thing, the unknown quantity being called the cosa,
      or the thing.]
      [1913 Webster]