וישמן ישורון ויבעט, "Israel became fat and kicked.⁠" Up until this time the Jewish people had never been called Yeshurun. It is possible that Moses did not use the name in order to flatter them, but, on the contrary, in order to point out how a people who were once considered entitled to such a compliment had forfeited their claim to that name. The name derives from אשורנו, a vision the prophet Bileam refers to as having had, i.e. the ability to behold supernatural phenomena (Numbers 14,17). The Jewish people had seen such phenomena, i.e. the attribute of כבוד at the time they experienced their spiritual high during the revelation at Mount Sinai. This is why Moses employs this term immediately after having described the Mount Sinai experience of the Jewish people with the words יסובבנהו יבוננהו in verse 10. Seeing that the Torah described the attribute Israel focused at that time as "and the appearance of the glory of God was similar to consuming fire at the top of the mountain,⁠" (Exodus 24,17) it is clear that the people at the time had a visual experience. Later on, during his blessings of the people in 33,4 Moses again refers to this term saying ויהי בישורון מלך, "the Lord became King amongst the people known as "Yeshurun.⁠" When Moses concludes his blessings of the people in Deut. 33,26 he again refers to the Jewish people as Yeshurun, saying אין כא-ל ישורון, "there is none like the God of Yeshurun.⁠" He meant that the very experience the people had had at Sinai should have convinced them that there is no power in the world that can equal their God.
All this is really part of this song which is a collection of criticism, rebuke, during which Moses argues with the people concerning their conduct in the future. He foretells them that they would become sinful and rebel against the Lord, becoming forgetful of all the kind deeds God had performed for them. This is why on this day, the last day of his life, he mentions the description Yeshurun three times to underline how terrible it is when a people who attained such spiritual stature as they would experience a spiritual decline.
The prophet Isaiah paraphrases Moses when he said in Isaiah 1,4 הוי גוי חוטא עם כבד עון, "woe, a sinful people, a nation laden with iniquity!⁠" Our sages comment on this that when Isaiah used the term גוי he referred to the people who had distinguished themselves as being referred to as ומי גוי גדול, "and who is such a great nation?⁠" How could such a nation become sinful? Furthermore, this was the nation which had been described as עם קדוש, "a consecrated people,⁠" in Deut. 14,2. How could such a people have fallen so much that it fits the description "laden with iniquity?⁠" The Jewish people had been described as "children are you to the Lord your God" in Deut. 14,1, and as "seed blessed by the Lord" (Isaiah 61,9).

ויטוש אלו-ה עשהו וינבל צור ישועתו, "it deserted God its Maker, and was contemptuous of the Rock of its salvation.⁠" The verse alludes to the two attributes of God which orchestrated the Exodus. The reason Moses did not say א-ל but אלו-ה was to include both the emanation תפארת and כבוד. This is why he added the letters ו and ה. This idea is reflected also in Job 35,10 איה אלוה עושי . The words וצור ישעותו mean that by means of the attribute צור, the deliverance of the Jewish people at the Sea of Reeds was orchestrated.