A latin expression, from Roman law. It is a form of term in a will. Usually it means that if you challenge the legality or terms of the will, you won't get any of the money / estate / objects that were originally designated to you or a signifcantly less part of it. The intention is to stop bickering over the will. However, if you choose to challenge the legality of the will and win, the will is declared dead along with ths clause.