The Collaborative International Dictionary of English v.0.48:

Cornage \Cor"nage\ (k?r"n?j), n. [OF.,, horn-blowing, tax on
   horned cattle, fr. F. corne a horn, L. cornu.] (Law)
   An ancient tenure of land, which obliged the tenant to give
   notice of an invasion by blowing a horn.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CORNAGE. The name of a species of tenure in England. The tenant by cornage
was bound to blow a horn for the sake of alarming the country on the
approach of an enemy. Bac. Ab. Tenure, N.