'Unspeakable' in the Bible

Did then a thing which is good become death to me? No, indeed, but sin did; so that through its bringing about death by means of what was good, it might be seen in its true light as sin, in order that by means of the Commandment the unspeakable sinfulness of sin might be plainly shown.

In the same way the Spirit [comes to us and] helps us in our weakness. We do not know what prayer to offer or how to offer it as we should, but the Spirit Himself [knows our need and at the right time] intercedes on our behalf with sighs and groanings too deep for words.