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]

WordNet (r) 3.0 (2006):

supposititious
    adj 1: based primarily on surmise rather than adequate evidence;
           "theories about the extinction of dinosaurs are still
           highly conjectural"; "the supposed reason for his
           absence"; "suppositious reconstructions of dead
           languages"; "hypothetical situation" [syn: conjectural,
           divinatory, hypothetical, hypothetic, supposed,
           suppositional, suppositious, supposititious]

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "supposititious":
   Barmecidal, Barmecide, accounted as, affected, airy, alleged,
   apocryphal, apparent, apparitional, artificial, assumed,
   assumptive, autistic, baseborn, bastard, bogus, brummagem,
   chimeric, chimerical, colorable, colored, conjectural, conjectured,
   counterfeit, counterfeited, deceptive, deemed, delusional,
   delusionary, delusive, delusory, dereistic, distorted, dreamlike,
   dreamy, dressed up, dummy, embellished, embroidered, erroneous,
   ersatz, factitious, fake, faked, fallacious, false, falsified,
   fancied, fanciful, fantastic, fatherless, feigned, fictional,
   fictitious, fictive, garbled, given, granted, hypothetical,
   illegitimate, illusional, illusionary, illusive, illusory,
   imaginary, imaginational, imagined, imitation, inferred, junky,
   make-believe, man-made, misbegotten, misleading, mock, natural,
   nonexistent, notional, ostensible, perverted, phantasmagoric,
   phantasmal, phantom, phony, pinchbeck, postulated, postulational,
   premised, presumed, presumptive, pretended, pseudo, put-on,
   putative, quasi, queer, reputed, seeming, self-deceptive,
   self-deluding, self-styled, sham, shoddy, simulated, so-called,
   soi-disant, specious, spectral, spurious, supposed, suppositional,
   suppositive, suppository, synthetic, taken for granted, tin,
   tinsel, titivated, twisted, unactual, unauthentic, understood,
   unfounded, ungenuine, unnatural, unreal, unsubstantial, visional,
   visionary, warped