Charles Ellicott Commentary Leviticus 3:9

"And he shall offer of the sacrifice of peace-offerings an offering made by fire unto Jehovah; the fat thereof, the fat tail entire, he shall take away hard by the backbone; and the fat that covereth the inwards, and all the fat that is upon the inwards," — Leviticus 3:9 (ASV)

The whole rump. — Better, the whole fat tail (Leviticus 7:3; Leviticus 8:25; Leviticus 9:19). The sheep of Syria and Palestine were, and still are, the bread-tailed species, the broad part often weighing fifteen pounds and upwards. In young animals, the substance of the tail, which consists of marrow and fat, tastes like marrow, and it is used by the Arabs for cooking instead of butter.

It is often so large that it trails on the ground, and to save the animal from the pain occasioned by dragging it on the ground, a small wheeled truck is attached to it, on which it draws it along. It is only the tail of sheep which is here included among the fat parts that are to be offered upon the altar. It is, however, not regarded as the ordinary fat of other quadrupeds (Leviticus 9:19), and hence, according to Jewish tradition, is not included in the prohibition of Leviticus 3:3.