A calf is a young cow. People would give one of their calves special food so that it would grow well, and then when they wanted to have a special feast, they would eat that calf. Alternate translation: "the best calf" or "the young animal we have been making fat" (See: [[rc://en/ta/man/translate/figs-explicit]])
The implied information that they were to cook the meat can be made explicit. Alternate translation: "kill it and cook it" (See: [[rc://en/ta/man/translate/figs-explicit]])