Open English Wordnet
Adjectives
(s)ineffable, unnameable, unspeakable, unutterabletoo sacred to be uttered“the ineffable name of the Deity”
(s)indefinable, indescribable, ineffable, unspeakable, untellable, unutterabledefying expression or description“indefinable yearnings”, “indescribable beauty”, “ineffable ecstasy”, “unspeakable happiness”, “unutterable contempt”, “a thing of untellable splendor”
(s)unpronounceable, unutterablevery difficult to pronounce correctly“an unpronounceable foreign word”, “unutterable consonant clusters”