too great or intense to be expressed; inexpressible
At Oxford he distinguished himself, to his father's ineffable satisfaction — Henry James
unspeakable
What ineffable twaddle! — Conan Doyle
too sacred to be uttered
His ineffable effable effanineffable deep and inscrutable singular name — T S Eliot
ineffability /-ʹbiliti/ noun
ineffably adv
[Middle English via French from Latin ineffabilis, from in-1 + effabilis capable of being expressed, from effari to speak out, from ex-1 + fari to speak]