Source : Webster's Revised Unabridged Dictionary (1913)

Subduplicate \Sub*du"pli*cate\, a. (Math.)
   Expressed by the square root; -- said of ratios.

   {Subduplicate ratio}, the ratio of the square roots, or the
      square root of a ratio; thus, the subduplicate ratio of a
      to b is [root]a to [root]b, or [root]a/b.