The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. He begins with tad, this, (the first word of the first hymn, tad id âsa). Verily 'this, this' is food, and thus he obtains food.
2. Pragâpati indeed uttered this as the first word, consisting of one or two syllables, viz. tata and tâta (or tat) 1. And thus does a child, as soon as he begins to speak, utter the word, consisting of one or two syllables, viz. tata and tâta (or tat). With this very word, consisting of tat or tatta, he begins.
3. This has been said by a Rishi (Rv. X, 71, 1) 2:--
4. 'O Brihaspati, the first point of speech;'--for this is the first and highest point of speech.
5. 'That which you have uttered, making it a name;'--for names are made by speech.
6. 'That (name) which was the best and without a flaw;'--for this is the best and without a flaw.
7. 'That which was hidden by their love, is made manifest;'--for this was hidden in the body, viz. those deities (which enter the body, Agni as voice, entering the mouth, &c.); and that was manifest among the gods in heaven. This is what was intended by the verse.
178:1 Tata and tâta are used both by children in addressing their parents, and by parents in addressing their children. If tat is called the very same word, eva is used in the sense of iva.
178:2 The verse is cited to confirm the meaning of tat, the first word of the first hymn (tad id âsa), as explained before. It was said that tat was the first name applied to a child. Now, according to Âsvalâyana Grihya-sûtra I, 16, 8, a name is given to a child at the time of its birth, a name which no one knows except father and mother, till the time when he is initiated by a Guru. This is called the abhivadanîya name. In allusion to this custom it is said here that tata is the secret name of the child, which becomes publicly known at a later time only. Of course the interpretation of the verse in that sense is unnatural, but quite in keeping with the general character of the Âranyaka. I doubt whether even the commentator understood what was intended by the author, and whether the gods who enter the body are supposed to know the name, or whether the name refers to these gods, or, it may be, to tad, the Brahman.