Jeshurun. This name, formed from יָשַׂר, righteous, designates Israel as chosen to be a righteous nation; and in the use of it here lies the keenest reproach of apostate Israel, as fallen into a state the opposite of that to which it was destined.
"By using the name righteous in place of Israel, Moses ironically censures those who had swerved from rectitude; by recalling to memory with what dignity they had been endowed, he the more sharply rebukes the perfidy which was their crime" (Calvin).
This name appears also in Deuteronomy 33:5, Deuteronomy 33:26, and in Isaiah 44:2; but in these places without any implied censure. By some the word is regarded as a diminutive from יָשׂוּר, the same as יָשָׂר, in the sense of rectulus, justulus, "the good little people" (Gesenius); others as a diminutive from יִשְׂרָאֶל, Israel, as a sort of term of endearment (Grotius).
But the latter of these derivations is impossible; and as to the former, there lacks evidence of the termination un having a diminutive significance in Hebrew. Besides, neither here nor in Deuteronomy 33:5 would a term of endearment be suitable.
Waxed fat, and kicked (cf. Deuteronomy 6:11; Deuteronomy 8:10; Deuteronomy 31:20). The allusion is to an ox that had grown fat through good feeding, and had become unmanageable in consequence (cf. 1 Samuel 2:26: Hosea 10:4).
Lightly esteemed. The Hebrew is strongly expressive here: Thou hast treated as a fool ( נִבֵּל, from נָבַל to be foolish (cf. Micah 7:6).