the GNU version of the Collaborative International Dictionary of English
adj. Incapable of being supposed; not supposable; inconceivable.
The Century Dictionary and Cyclopedia
Not supposable; incapable of being supposed.
Word Usage
"That fine old Fabius, General Scott, had no idea of flinging an army out broadcast into Virginia, and, in the insupposable case that it turned tail, leaving it no defended passage to run away by."