in·ef·fa·ble 
(ĭn-ĕfə-bəl)
(ĭn-ĕfə-bəl)adj.
1. Incapable of being expressed; indescribable or unutterable: ineffable joy.
2. Not to be uttered; taboo: the ineffable name of God.
[Middle English, from Old French, from Latin ineffābilis : in-, not; see IN-1 + effābilis, utterable (from effārī, to utter : ex-, ex- + fārī, to speak; see bhā-2 in the Appendix of Indo-European roots).]
in·ef′fa·bili·ty, in·effa·ble·ness n.
in·effa·bly adv.