Inexpressible
Part of speech: adjective
Definition: Unable to be expressed.
Example sentence: There, in the chords and melodies, is everything I want to say. The words just jolly it along. It's always been my way of expressing what, for me, is inexpressible by any other means.
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: Look at the world of both film and indie games, and you'll find a startling similarity between the two when it comes to creating the perfect horror story. The tricks storytellers pull to make your blood run cold never change; a creaking floorboard, the eerie feeling of being watched, wandering into a world filled with unspeakable terror.