Definition: Adjective. Something that is so great or wonderful that it cannot be described with words.
Etymology: From the Latin ineffabilis meaning “unutterable;” lietrally, “not speakable from in- “not” and effabilis “speakable.”
Related words: ineffably (adverb) and ineffability (noun).
For further reading: Oxford English Dictionary, 2nd Edition edited by John Simpson and Edmund Weiner, Oxford University Press (1991).