Source : Webster's Revised Unabridged Dictionary (1913)

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.]