The allusion is to the New Testament story of the prodigal son (Luke 15 :11 - 32) in which the forgiving father orders his best calf to be killed in order to provide a feast to celebrate the return of his wayward son. Fatted is an archaic form of the verb fat meaning MAKE or BECOME FAT. Nowadays we use the forms fatten and fattened.
Related Idioms :
a golden calf
something, especially wealth, as an object of excessive or unworthy worship
In the Bible, the golden calf was a statue of gold in the shape of a calf, made by Aaron in response to the Israelites' plea for a god while they awaited Moses' return from Mount Sinai where he was receiving the Ten Commandments (Exodus - chapter 32).