Short for neminem captivabimus nisi iure victum, (Latin, "We shall not arrest anyone without a court verdict").
In the Crown of the Kingdom of Poland and the Polish–Lithuanian Commonwealth it was one of the basic rights, stating that the king could neither punish nor imprison any member of the szlachta without a viable court verdict. Its purpose was to release someone who had been arrested unlawfully. Neminem captivabimus had nothing to do with whether the prisoner is guilty, only with whether due process had been observed.
It was introduced by king Władysław Jagiełło in the acts of Jedlnia (1430) and Kraków (1433) and remained in use until the Partitions of Poland (1772-1795). The same acts guaranteed, that he shall not confiscate any szlachta property without a court verdict.