The Collaborative International Dictionary of English v.0.48:

Unconceivable \Un`con*ceiv"a*ble\, a.
   Inconceivable. [Obs.] --Locke. -- Un`con*ceiv"a*ble*ness,
   n. [Obs.] -- Un`con*ceiv"a*bly, adv. [Obs.]
   [1913 Webster]