Complex sentence: Serjeanty originated in the assignation of an estate in land on condition of the performance of a certain duty other than knight-service, usually the discharge of duties in the household of king or noble.
Split into simpler sentences: Serjeanty originated in the assignation of an estate in land on condition of the performance of a certain duty other than knight-service. Its essence might be described as "servantship"-the discharge of duties in the household of king or noble.