Where does "Yeshurun" come from?

Question

Am Yisrael has a number of names in Tanach. "Yaakov" (such as "Beit Yaakov") and "Yisrael" ("Bnei Yisrael") come from the names of Yaakov Avinu. "Yehudim" from the Kingdom of Yehudah. And so forth. I was wondering where the name "Yeshurun" came from? It first appears in Devarim 32:15:

"So Jeshurun grew fat and kicked"

and everyone takes it for granted that it refers to Yisrael - but it's kind of like there's a scene missing (ח"ו) in the Torah during which Am Yisrael were bequeathed this title by someone. Where did it come from? How did it come to refer to Am Yisrael? It doesn't seem like it was invented then and there by Moshe or Hashem.

Answer 1

Where does "Yeshurun" come from?

Is this not written in the סֵ֣פֶר הַיָּשָׁ֑ר Scroll of The-Upright? [Referencing Yehoshua 10:13-14]

The term "Yeshurun" יְשֻׁרוּן֙ comes from יָּשָׁ֑ר "Yashar" which Yehoshua referenced when writing his edition of Devarim.

The Tanakh reveals in [Joshua 8:32] that the completed scroll of Deuteronomy (Devarim, דְּבָרִ֗ים) was written by Joshua (Yehoshua, יְהוֹשֻׁ֙עַ֙).

The modern text of Deuteronomy is based on Joshua's copy of the original scroll written by Moses (Moshe, משֶׁה֙).

[Yehoshua | Joshua 8:32] "And he wrote there upon the stones a copy of the Torah of Moshe, which he wrote in the presence of the children of Yisrael." (וַיִּכְתָּב־שָׁ֖ם עַל־הָֽאֲבָנִ֑ים אֵ֗ת מִשְׁנֵה֙ תּוֹרַ֣ת מֹשֶׁ֔ה אֲשֶׁ֣ר כָּתַ֔ב לִפְנֵ֖י בְּנֵ֥י יִשְׂרָאֵֽל)

Joshua's edition of [Devarim 33:4] helps give credit to Moses for writing the Torah.

[Devarim | Deuteronomy 33:4] "Torah that Moshe commanded us is a legacy for the congregation of Yaqov." (תּוֹרָ֥ה צִוָּה־לָ֖נוּ משֶׁ֑ה מֽוֹרָשָׁ֖ה קְהִלַּ֥ת יַֽעֲקֹֽב )

The legacy term "Yeshurun" יְשֻׁרוּן֙ comes from יָּשָׁ֑ר "Yashar" when Yehoshua referenced the סֵ֣פֶר הַיָּשָׁ֑ר Scroll of The-Upright.

Additionally terms like "Yesharim" refer not only to Yaqov (but also Avraham & Yitschaq) as stated in [Avodah Zarah 25a.1] :

"The Gemara cites a series of expositions with regard to what is called the book of Yashar. The verse states: “And the sun stood still, and the moon stayed, until the nation had avenged themselves of their enemies. Is this not written in the book of Yashar”? (Joshua 10:13). The Gemara asks: What is the book of Yashar? Rabbi Ḥiyya bar Abba says that Rabbi Yoḥanan says: This is Genesis, which is the book of Abraham, Isaac, and Jacob, who were called righteous [yesharim], as it is stated: “Let me die the death of the righteous [yesharim]” (Numbers 23:10)." ( (יהושע י, יג) וידום השמש וירח עמד עד יקום גוי אויביו הלא היא כתובה על ספר הישר מאי ספר הישר א"ר חייא בר אבא א"ר יוחנן זה ספר אברהם יצחק ויעקב שנקראו ישרים שנא' (במדבר כג, י) תמות נפשי מות ישרים )

When did "Yeshurun" become a term for the Yesharim?

[ Yoma 37b.8 ] "The letter tzadi is not written within the names of the twelve tribes engraved on the breastplate’s stones. Rav Shmuel bar Yitzḥak said: The names Abraham, Isaac, and Jacob were also written there. The name Yitzḥak, Isaac, contains the letter tzadi. The Gemara asks again: But surely the letter tet was not written on the breastplate, since it is not found in the names of the Patriarchs nor in the names of the twelve tribes. Rav Aḥa bar Ya’akov said: Shivtei Yeshurun, [the Tribes of Jeshurun], was also written there. The word shivtei, tribes, contains the letter tet. In this way the entire alphabet was represented."

[Source: https://www.sefaria.org/Yoma.73b.8?with=all&lang=bi]