I have read that aishdas (Deut. 33:2) means "fiery law" or something of the sort. However, doesn't the word das with the meaning of law/religion come from Persian (i.e. did not have the meaning of law in Biblical times)? Can someone help me out here?
Prof Richard Steiner argues in this article that דת here is short for דאת, a rare verb meaning "fly" (like Deu 28:49 כאשר ידאה הנשר, but in an archaic construction like Lev 25:21 ועשת and with the א elided). So the meaning of the verse is really "from His right hand, fire flew to them," and the commonly seen "fiery law" is based on Midrash. Indeed given the structure of the verse a verb like that is to be expected here. He and Rabbi Dr. Sid Z. Leiman in this article find support for this in Targum Psuedo-Jonathon to Exodus 20:2 which, in parallel to Sifre on our verse, explicitly describes at the Revelation at Sinai a torch/fire flying (טייס -- the same root the Targum uses to translate ידאה) from God's right hand to [surround] the Jewish people.
In defense of the more common translation, one might argue that דת as "law" may be a native Hebrew word that is otherwise unattested, rather than a loanword from Persian.
Based on the k'siv, in which "אשדת" is only one word, I have heard it being related to the independent word אשדת, meaning waterfall, as in "תחת אשדת הפסגה". (D'varim 4:49)
Conceivably, דת is simply a semitic root that happens to only come up once in the earlier Hebrew of the Chumash, but was used more in Persian. It appears in Aramaic and Syriac as דתא (which is also found in the book of Ezra).
But yes, the kesiv is "waterfall".
A nice metaphor for Torah -- fiery passion and rite fused into a whole that is neither, from which flows life.
The Persian language did not exist at the time of Torah? Not even a prototype?
I'm not sure I really understand the difficulty. The Torah has words that were not (originally) Hebrew. A couple of examples off the top - Totofos (tefilin) and Moshe.
Is there anywhere stated that all the words in the Torah had to be of Hebrew origin?
According to Rash"i (on B'reshis 37:17), Ya'akov's other ten sons used the word דת on their way to plot against Yosef.