I have found an article that explain such a meaning... See above:
The genuine meaning of Tathagata
Copyright 2007 Aryasatvan
The Tathagata, pronounced: “Taaht-ahgatah”, in the common nonsensical definition by ignorant modern “Buddhism” is meant “thus come one”, or “thus gone one”. This view ignorantly implies a formal appellation of importance (such as Sir, Master, Great-One, etc.) rather than a denotation of a profound spiritual attainment.
The term Tathagata is composed of two parts, Tat, and agata. Tat has been since time immemorial in India, meant Brahman, the Absolute, as in the famous Upanishadic dictum: “That (Brahman) thou art” (tat tvam asi). “That” is here, of course Brahman, the Godhead, the Subject of Selfhood which the muni, or sage, has reached at the pinnacle of his having fulfilled wisdom’s perfection. Agata is the past tense denotation of gata (going, traveling, trekking), here being meant “arrival, gone-unto, attainment of, arrival-at”. As such, Tathagata in the ancient Prakrit Pali, is meant literally “(The sage who has) arrived at the Absolute”, or in Sramanic context of Vedanta and Buddhism, “(He-thou) is (arrived at) That”. The very term Tathagata, which has of yet never been discovered by anyone until now, is none other than a personal appellation of that very rare someone who has realized by wisdom “tat tvam asi”. The Tathagata, therefore, is equally as well meant “The ‘tat tvam asi’ comprehensor/sage”.
It is unfathomable that modern so-called Buddhism’s position is that the spiritual appellation of the Buddha’s attainment, “attained/arrived at Brahman” (Tathagata) is merely an honorary designation for a popular sage. As [It 57] and other passages clearly show, “become-Brahman” is the meaning of the term Tathagata, or he who has arrived (agata), again being meant the transfiguration and assimilation of the mind (citta) in upon itself (bhava), and thereby achieving the Absolute, i.e. Brahman, as such (brahmabhutam tathagata) is said. To say that Tathagata, is meant by nonsensical “Buddhism”, to the effect: that Tathagata denotes the “thus-come one”, or “thus-gone one” has no contextual validity, is utterly illogical to read Pali as such, and carries no meaning whatsoever, which is all the more so magnified given that the very term Tathagata carries, regardless of translation, a very weighty importance and denotation; thereby secular ‘Buddhism’ intends to castrate the meaning of the term Tathagata, is yet another resection of original Buddhism by modern sects to turn Buddhism into a moralistic movement devoid of metaphysics.
Scriptural collaboration of same: (Tathagatassa hetam, adhivacanam brahmabhuto itipi)-“The Tathagata means 'the body of Brahman', 'become Brahman'” [DN 3.84]. (brahmabhutam tathagata)-“Become-Brahman is the meaning of Tathagata” [It 57]. Many more such passages are preset in suttana.
Source: Aryan Buddhism Blog
I do not have any schoolar knowledge, so this article get me confused. The autor of the blog also made some heavy critics about moderm buddhism culture, even about Theravada.
Any one could clarify the Therada version for this account?