Ineffable
Part of speech: adjective
Definition: Beyond expression in words; unspeakable. Forbidden to be uttered; taboo.
Example sentence: In certain strains of Judaism, there's a profound passion for the ineffable. Contemplation of God is meant to be forever elusive, because, you know, our tiny minds can't possibly comprehend Him. If we find ourselves comprehending Him, then we can be sure we're off track.
Untellable
Part of speech: adjective
Definition: indefinable, indescribable or ineffable