1. （尤指不好的事情）无以名状的，难以形容的，莫名的
You can use unutterable to emphasize that something, especially a bad quality, is great in degree or intensity.

1. very difficult to pronounce correctly

e.g. an unpronounceable foreign word
unutterable consonant clusters

Synonym: unpronounceable

2. defying expression or description

e.g. indefinable yearnings
indescribable beauty
ineffable ecstasy
inexpressible anguish
unspeakable happiness
unutterable contempt
a thing of untellable splendor

Synonym: indefinableindescribableineffableunspeakableuntellable

3. too sacred to be uttered

e.g. the ineffable name of the Deity

Synonym: ineffableunnameableunspeakable