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Prodigal son
Public DomainProdigal son - Credit: Rembrandt van Rijn

A fatted calf is a metaphor for the festive celebration of someone's long-awaited return. It derives from the parable of the prodigal son, in the New Testament of the Bible. 

In biblical times, people would often keep at least one animal that was fed a special diet to fatten it up, in preparation for a special occasion.

In the Bible story, when the prodigal son returns, his father kills the fatted calf to demonstrate how special the resulting celebration is.