Insupposable

In`sup·pos'a·ble adjective Incapable of being supposed; not supposable; inconceivable.
Found on http://www.encyclo.co.uk/webster/I/73

Insupposable

• (a.) Incapable of being supposed; not supposable; inconceivable.
Found on http://thinkexist.com/dictionary/meaning/insupposable/
No exact match found