1. Incapable of being expressed; indescribable or unutterable. See Synonyms at unspeakable.
2. Not to be uttered; taboo: the ineffable name of God.
[Middle English, from Old French, from Latin ineffbilis : in-, not; see in-1 + effbilis, utterable (from effr, to utter : ex-, ex- + fr, to speak; see bh-2 in Indo-European roots).]