This is an allusion to the story of the Prodigal Son, as told in the Gospel of Luke (15:11-32).
The Prodigal Son asks his father to give him his inheritance early, and swiftly squanders it all. Reduced to absolute poverty, and after living in a pig pen for a while, the son returns home expecting his father to be angry. Instead the father throws a lavish party in celebration of his son's return, killing a fatted calf in his honour.
'prodigal' means 'recklessly extravagant'.