Brewer's: Augean Stables
The stables of Augeas, King of Elis, in Greece. In these stables he had kept 3,000 oxen, and the stalls had not been cleansed for thirty years. When Hercules was appointed to cleanse these stables, he caused two rivers to run through them.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894