Charles Ellicott Commentary Deuteronomy 32:15

"But Jeshurun waxed fat, and kicked: Thou art waxed fat, thou art grown thick, thou art become sleek; Then he forsook God who made him, And lightly esteemed the Rock of his salvation." — Deuteronomy 32:15 (ASV)

Jeshurun is a diminutive—a term of endearment. Either “the child of the upright,” or “the beloved Israel.” The letters of the diminutive of Israel, if slightly abbreviated, would make “Jeshurun.” It is peculiar to Deuteronomy (here and in Deuteronomy 33:5; Deuteronomy 33:26) and Isaiah (Isaiah 44:2). Two of the Targums render the word by “Israel here.” The third retains the word itself. The Septuagint translate it “the beloved one.”

Kicked. —Only in 1 Samuel 2:29: Wherefore kick you at my sacrifice and my offering ... to make yourselves fat?

Grown thick. —As Rehoboam said, My little finger shall be thicker than my father’s loins. Both these parallels illustrate the spirit of the verse.