The Collaborative International Dictionary of English v.0.48:

Escuage \Es"cu*age\ (?; 48), n. [OF. escuage, F. ['e]cuage, from
   OF. escu shield, F. ['e]cu. See Esquire.] (Feud. Law)
   Service of the shield, a species of knight service by which a
   tenant was bound to follow his lord to war, at his own
   charge. It was afterward exchanged for a pecuniary
   satisfaction. Called also scutage. --Blackstone.
   [1913 Webster]

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

ESCUAGE, old Eng. law. Service of the shield. Tenants who hold their land by
escuage, hold by knight's service. 1 Tho. Co. Litt. 272; Littl. s. 95, 86 b.