An unutterable feeling is so strong, so extreme, that words cannot hold it. Literally it means "cannot be spoken" — utter means to say something out loud. But nobody uses it to mean your mouth doesn't work. It is used to say: this feeling was too big for language. Unutterable joy, unutterable sadness, unutterable loneliness. So it works as a very strong "extremely". Notice the trick inside it, though: the writer says the feeling cannot be described — and by saying that, describes how enormous it is. That is why writers like it. It is a way of pointing at something instead of explaining it. This is a literary, old-fashioned word. You will find it in novels, poems, hymns and speeches, not in a message to a friend. If a student today wants to say a feeling was huge, natural English is "I can't describe how happy I was" or "it was indescribable". Say "unutterable happiness" at lunch with friends and you will sound like a character in a book — which is sometimes exactly the joke people are making. There is a second, older use worth knowing because you meet it in religious writing and in horror stories: something unutterable is something that must not be said, because saying it is forbidden or too terrible. An unutterable name; unutterable horrors. Same shape, different reason — the first means language is too small, the second means saying it is not allowed.

難以言喻的、說不出口的（形容感受強烈到無法用言語形容；文學用語）