Dictionary.net

• An ancient and now almost obsolete remedy in the English law. An assi/.e of mort d'anccstor was a writ which lay for a person whose ancestor died seised of lands in fee-simple, and after his death a stranger abated ; and this writ di- rected the sheriff to summon a jury or assize, who should view tbe land in question and recognize whether such ancestor were seised thereof on the day of his death, and whether the demandant were the next heir.

1910 - Black's Law Dictionary (2nd edition)
By Henry Campbell Black