Rashi on Leviticus 3:4/רש"י על ויקרא ג׳:ד׳

1הכסלים — Flancs in O. F. — It speaks of the fat which is on (על) the kidneys being also on (על) the flanks because the fat that is upon the kidneys is, when the animal is living, on the uppermost part of the flanks, and they (the kidneys) are in a downward position. This is the fat which is under the loins — what are called lombles in O. F.; it is that white fat which is visible upon the upper part of the flanks, but on their lower part the flesh covers it, (and this latter portion of the fat is therefore not upon but under the flanks).

2היותרת is the protecting wall (membrane) over the liver which is called abris in O. F. In the Aramaic language it is called חצרא דכבדא the lobe of the liver.

3על הכבד — This must mean that he shall take a little of the liver with it, for in another place it distinctly states, (Leviticus 9:10) “and the יותרת מן הכבד [he caused to ascend in fumes]” (Sifra, Vayikra Dibbura d'Nedavah, Section 14 8).

4על הכבד על הכליות — means, besides the part of the liver and besides the kidneys shall he remove this (the יותרת‎).