incognoscible

from The Century Dictionary.

• Not cognoscible; such as cannot be known or recognized; incognizable.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Incognizable.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective rare, dated Not capable of being known; incomprehensible.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective incapable of being perceived or known