Emily Dickinson Lexicon

MORT'MAIN, n. [Fr. mort, dead, and main, hand.]

In law, possession of lands or tenements in dead hands, or hands that can not alienate. Alienation in mortmain is an alienation of lands or tenements to any corporation, sole or aggregate, ecclesiastical or temporal, particularly to religious houses, by which the estate becomes perpetually inherent in the corporation, and unalienable. Blackstone.