[adj] very difficult to pronounce correctly; "an unpronounceable foreign word"; "unutterable consonant clusters"
[adj] defying expression or description; "indefinable yearnings"; "indescribable beauty"; "ineffable ecstasy"; "inexpressible anguish"; "unspeakable happiness"; "unutterable contempt"; "a thing of untellable splendor"
[adj] too sacred to be uttered; "the ineffable name of the Deity"