ineffable

Etymology

From Middle French ineffable (modern French ineffable), from Latin ineffābilis, from in- (“not”) + effor (“utter”) + -bilis (“-able”).

adj

1. Beyond expression in words; unspeakable.
   Devotion bids aspire to nobler things, to boundless love, and joys ineffable: and such her expectation from kind Heav'n. 1750, “Theodora”, Thomas Morell (lyrics), George Frideric Handel (music)
   God does not play dice with the universe; He plays an ineffable game of his own devising, which might be compared, from the perspective of any of the other players, to being involved in an obscure and complex version of poker in a pitch dark room, with blank cards, for infinite stakes, with a dealer who won't tell you the rules, and who smiles all the time. 1990, Terry Pratchett, Neil Gaiman, Good Omens
   As Alan Watts (1961) wrote, it involves trying to speak the unspeakable, scrute the inscrutable and eff the ineffable. 2012, Gay Watson, Stephen Batchelor, Guy Claxton, The Psychology of Awakening: Buddhism, science, and our day-to-day lives
2. Forbidden to be uttered; taboo.