The first intuition of Shūnyam originates in the word Tat [literally, That] in the Rig Veda [see: SRA]
That is an ‘Expression of Inexpressibility’, a self-scuttling assertion in negation, an immediate, unregenerate self-contradiction. It is neither noun nor verb, is grammatically homeless, a lexicographer’s nightmare, and meant to be so.
You cannot express it; if you can express it, what you have expressed is not it.
That is a Self-Eating Expression [‘SEE’]. And the birth of the intuition of Shūnyam.
By 500 BCE, Tat [That] had settled into a firmly expressed idea. The most consequential of them was as ‘Inner Subtle Essence’ [which first makes itself known in the Chandogya Upanishad].
This mysterious Essence soon took on a life of its own and quickly became an ‘Inner Being’, and in time the Vedanthic ‘Self’, a cornerstone of contemporary understanding.
It was clear to the observing Scholar-Monks that a new term be proffered, one that would be true to the original intended meaning of That.
See: ‘Yagnavalkya’s Rule‘