You were right. TATHATA means thusness, suchness, just-like-that-ness: neither this nor that, the reality of non-duality. Things are just as they are (void and dependently originated) regardless of our perceptions, likes and dislikes.
The three words “tathata”,”avitathata” and “anannatatha” are very important. Also the word “Tathagata”. Tathagata is a descriptive term used by the Supreme Buddha in referring to himself. As a name for a Buddha, it means the Being Par-excellence, the Great Being. Gautama Buddha frequently in the Suttas speaks of himself as the Tathagata. We can see many contexts where the term Tathagata has been mentioned in the first four Nikayas. Tathagata means the perfect one. Literary the one who has “thus gone” or “thus come”. It is an epithet of the Buddha used by him when speaking of himself. The term has derived from “tatha” or “tatha” in pali, which means “thus” and “truth” or “reality”. So when the term taken with the suffix “gata”(Tatha + gata) provides the meaning “thusgone” and when the term is made by the suffix “agata” (Tatha + agata) provides the meaning “thus come”.
Since the term is related to truth “Tathata” yields the meaning thatness or suchness literary “reality”. Avitathata means that the truth that can not be changed and proved in its nagative aspect. Anannata is that one can not provide alternative of suchness or truth. So the three terms indicate some characteristics of reality. Therefore in term “Tathagata” “tatha” means reality or the truth .That is Nibbana. Accordingly those who have come or gone to the state of reality or Nibbana are “Tathagatas”.