An inexpressible feeling is too strong to put into words. Joy, grief, love, relief — the big ones. It is a literary word. You will see it in books and careful writing more than in a chat. People do not usually use it for small things like being a bit hungry. Though actually, writers sometimes pile it on (“inexpressible beauty”) as a high compliment. That use is a little stretched, but you will meet it in older, grand prose. The word is about a feeling (or a quality) that language cannot hold, not about a face that shows nothing.

難以言喻的、無法形容的（偏文學、書面）