E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Festus says the Romans called victims wholly consumed by fire prodig hosti (victims prodigalised), and adds that those who waste their substance are therefore called prodigals. This derivation can hardly be considered correct. Prodigal is pro-ago or prod-igo (to drive forth), and persons who had spent all their patrimony were driven forth to be sold as slaves to their creditors.
Prodigal (The). Albert VI., Duke of Austria. (14181463.)