Ineffable
Part of speech: adjective
Definition: Beyond expression in words; unspeakable. Forbidden to be uttered; taboo.
Example sentence: People say that I am always serious and depressing, but it seems to me that the English are never serious - they are flippant, complacent, ineffable, but never serious, which is sometimes maddening.
Inexpressible
Part of speech: adjective
Definition: Unable to be expressed.
Example sentence: What is modern art but the attempt to pinpoint vague, incorporeal, inexpressible sensations? What is modern art, I would add, but the most solemn pile of nonsense that ever appeared on Earth?