Charles Ellicott Commentary Deuteronomy 32:14

"Butter of the herd, and milk of the flock, With fat of lambs, And rams of the breed of Bashan, and goats, With the finest of the wheat; And of the blood of the grape thou drankest wine." — Deuteronomy 32:14 (ASV)

Kidneys of wheat. —The metaphor is literally translated from the Hebrew. The kidneys are enclosed in the very best of the fat of the animal, fat that was strictly reserved for God’s altar by the Levitical Law.