The Collaborative International Dictionary of English v.0.48:

Imposing \Im*pos"ing\, n. (Print.)
   The act of imposing the columns of a page, or the pages of a
   sheet. See Impose, v. t., 4.
   [1913 Webster]

   Imposing stone (Print.), the stone on which the pages or
      columns of types are imposed or made into forms; -- called
      also imposing table.
      [1913 Webster]