What is meant by cleaning the Augean stables?

Augeas ‘(pronounced aw-j’ee-us) was a legendary king of Elis in Greece who possessed a herd of three thousand sacred oxen whose stalls had not been cleaned for thirty years. The cleaning of these stables in a single day was the sixth of the twelve difficult and dangerous labors imposed upon Hercules by Eurystheus, King of Argolis. Hercules, who was promised a tenth part of the cattle in payment, succeeded in performing the task within the required time by turning the rivers Alpheus and Peneus through the stables. Augeas refused to turn over one-tenth of the oxen to Hercules on the ground that he had carried out the commission only in the service of Eurystheus, whereupon Hercules sent an army into Elis and slew Augeas and his sons. Cleaning or purging the Augean stables means to clean away an accumulated mass of corruption or filth. Figuratively, he who reforms abuses almost past the power of man to remedy is said to clean the Augean stables.