The Collaborative International Dictionary of English v.0.48:

Subduplicate \Sub*du"pli*cate\, a. (Math.)
   Expressed by the square root; -- said of ratios.
   [1913 Webster]

   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.
      [1913 Webster]