The Collaborative International Dictionary of English v.0.48:

Sergeanty \Ser"geant*y\, n. [Cf. OF. sergentie, LL. sergentia.
   See Sergeant.] (Eng. Law)
   Tenure of lands of the crown by an honorary kind of service
   not due to any lord, but to the king only. [Written also
   serjeanty.]
   [1913 Webster]

   Grand sergeanty, a particular kind of tenure by which the
      tenant was bound to do some special honorary service to
      the king in person, as to carry his banner, his sword, or
      the like. --Tomlins. --Cowell. --Blackstone.

   Petit sergeanty. See under Petit.
      [1913 Webster]

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

SERJEANTY, Eng. law. A species of service which cannot be due or performed
from a tenant to any lord but the king; and is either grand or petit
serjeanty.