Unspeakable
Part of speech: adjective
Definition: Incapable of being spoken or uttered; unutterable; ineffable; inexpressible. Unfit or not permitted to be spoken or described. Extremely bad or objectionable.
Example sentence: Our country's national crime is lynching. It is not the creature of an hour, the sudden outburst of uncontrolled fury, or the unspeakable brutality of an insane mob.
Inexpressible
Part of speech: adjective
Definition: Unable to be expressed.
Example sentence: Our job as writers, as far as I can tell, is to attempt to express what seems inexpressible.