It looks more like the fourth principle part of a verb, either sago, sagere, segi, sactum on the model of ago, agere, egi, actum or sacio, sacere, seci, sactum on the model of facio, facere, feci, factum but I can't find anything for either of those WAGs. The closest I come is sacrum which could mean accursed if dux were neuter.