knight's service

CIDE DICTIONARY

knight's service, n. 

•  The military service by rendering which a knight held his lands.
•  A tenure of lands held by knights on condition of performing military service. See Chivalry, n., 4.  [1913 Webster]

  "By far the greater part of England [in the 13th century] is held of the king by knight's service. . . . In order to understand this tenure we must form the conception of a unit of military service. That unit seems to be the service of one knight or fully armed horseman (servitium unius militis) to be done to the king in his army for forty days in the year, if it be called for. . . . The limit of forty days seems to have existed rather in theory than practice."  [Webster 1913 Suppl.]
•  Service such as a knight can or should render; hence, good or valuable service.  [Webster 1913 Suppl.]