Albert Barnes 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 - The whole fat tail: that is, the tail of the kind of sheep well known in the East, and often weighing 15 lbs. and even as much as 50 lbs., when the sheep has been increased by artificial fattening.