mortmain
The perpetual, inalienable possession of lands by a corporation or non-personal entity such as a church.
uncountable
[W]e do hereby grant our especial license and authority unto all and every person […] to grant sell alien and convey in mortmain unto and to the use of the said Society and their successors […]
1824, Charter of Incorporation of the Royal Asiatic Society of Great Britain and Ireland,A strong and inalienable possession.
literary, uncountable
[…]; and some part of that influence [of the government], which would otherwise have been possessed as in a sort of mortmain and unalienable domain, returned again to the great ocean from whence it arose, […]
Where it comes from
From Anglo-Norman mortmayn, morte meyn, from Old French mortes meins, after Late Latin phrase mortua manus. See Latin mortuus (“dead”) + manus (“hand”).
Adapted from the Wiktionary entry mortmain (CC BY-SA 4.0).