the appendage of the liver: the projecting part removed with the fat and burned on the altar
This is a technical term from Israel's sacrificial law. It appears only in the instructions for offerings in Exodus and Leviticus, and always in the same setting: this part of the liver is lifted off together with the two kidneys and the surrounding fat and turned to smoke on the altar as the portion given to God.
The name comes from a root meaning "to remain over" or "stick out," so the word points to the part that projects, the appendage of the liver rather than the whole organ. Anatomically this is the small, finger-shaped lobe on the underside of the liver.
Because it names one fixed body part in one fixed ritual, the word never develops other senses. Its difficulty is not that it means many things but that English has no single, familiar word for this exact piece of anatomy.