Incapable of being expressed in words; unspeakable; unutterable; indescribable; as, the ineffable joys of heaven.

defying expression or description; "indefinable yearnings"; "indescribable beauty"; "ineffable ecstasy"; "inexpressible anguish"; "unspeakable happiness"; "unutterable contempt"; "a thing of untellable splendor"

too sacred to be uttered; "the ineffable name of the Deity"

Incapable of being expressed; indescribable or unutterable. Not to be uttered; taboo: the ineffable name of God.

Too awesome or sacred to be spoken, inexpressible, or too overwhelming to be described in words -- said of God's name, "I AM" (YHWH, the Tetragrammaton), as expressed to Moses in Exodus 3:14.