pre-noun adjectival
indescribable; inexpressible; unspeakable
Used before nouns to describe something so profound, emotional, or vague that it cannot be put into words. Often found in literary or emotional contexts.
I was seized by an indescribable anxiety.
His words have an inexpressible charm.
From the negative form of the classical verb 言い知る (to know by speaking), literally 'cannot be known through words'. The modern form is a fixed pre-noun adjectival.