labor of Hercules

A very difficult task. When the Greek hero Hercules was driven mad because of the goddess Hera's jealousy, he murdered his children. As atonement for his crime, he was obliged to perform twelve demanding tasks, such as slaying or capturing dangerous beasts, obtaining various prized and well-guarded possessions, and cleaning a very dirty stable in just one day. Hercules succeeded and was granted immortality and the hand of the now-mollified Hera's daughter. If your boss gives you an impossible assignment, especially that must be completed in a short time, you could show off your classical education by referring to it as a labor of Hercules.