The Collaborative International Dictionary of English v.0.48:
Esnecy \Es"ne*cy\, n. [See Eigne.] (Eng. Law) A prerogative given to the eldest coparcener to choose first after an inheritance is divided. --Mozley & W. [1913 Webster]Bouvier's Law Dictionary, Revised 6th Ed (1856):
ESNECY. Eldership. In the English law, this word signifies the right which the eldest coparcener of lands has to choose one of the parts of the estate after it has been divided.