The Collaborative International Dictionary of English v.0.48:

Supposititious \Sup*pos`i*ti"tious\, a. [L. suppositicus. See
   Supposition.]
   1. Fraudulently substituted for something else; not being
      what is purports to be; not genuine; spurious;
      counterfeit; as, a supposititious child; a supposititious
      writing. --Bacon.
      [1913 Webster]

   2. Suppositional; hypothetical. [R.] --Woodward.
      [1913 Webster] -- Sup*pos`i*ti"tious*ly, adv. --
      Sup*pos`i*ti"tious*ness, n.
      [1913 Webster]