The Old Law was an old Hutt law that allowed for challenges between clan leaders if sufficient provocation had occurred. It allowed for one-on-one mortal combat.
After the destruction of the Hutt's homeworld of Varl, a radical change occurred in Hutt culture. It would henceforth be forbidden by law, under pain of the most extreme of punishments (ostracism, bankruptcy, death), for a Hutt to kill another Hutt. The Hutt culture then evolved from military domination to economic domination, and it was seen as proper to ruin an opponent rather to kill her.
The "Old Law" is the only known exception to aforementioned law. It allows the recognized leader of a Hutt kajidic to challenge the recognized leader of another kajidic to personal one-on-one mortal combat, in the case sufficient provocation has been offered by the challengee to the challenger. The victor of the duel would be recognized as having right on his side, and would not be prosecuted for killing a fellow Hutt.
Although legal and accepted, most Hutts frowned on challenges under the Old Law, since it resulted in the death of one of their most valuable leaders, which was perceived as a waste.