Fatted calf is a metaphor or symbol of festive celebration and rejoicing for someone's long-awaited return. It derives from the parable of the prodigal son in the New Testament. In biblical times, people would often keep at least one piece of livestock that was fed a special diet to fatten it up, thus making it more flavorful when prepared as a meal. Slaughtering this livestock was to be done on rare and special occasions. Thus when the prodigal son returns, the father "kills the fatted calf" to show that the celebration is out of the ordinary.
In modern usage, "killing the fatted calf" can simply mean to celebrate in an exuberant manner, as used in the Elton John song Bennie and the Jets.
Famous quotes related to fatted calf:
“And get the fatted calf and kill it, and let us eat and celebrate; for this son of mine was dead and is alive again; he was lost and is found!’ And they began to celebrate.”
—Bible: New Testament, Luke 15:23,24.
“But he answered his father, Listen! For all these years I have been working like a slave for you, and I have never disobeyed your command; yet you have never given me even a young goat so that I might celebrate with my friends. But when this son of yours came back, who has devoured your property with prostitutes, you killed the fatted calf for him!’”
—Bible: New Testament, Luke 15:29,30.