The Collaborative International Dictionary of English v.0.48:

Struck \Struck\,
   imp. & p. p. of Strike.
   [1913 Webster]

   Struck jury (Law), a special jury, composed of persons
      having special knowledge or qualifications, selected by
      striking from the panel of jurors a certain number for
      each party, leaving the number required by law to try the
      cause.
      [1913 Webster]

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

STRUCK JURY. A special jury selected by striking from the panel of jurors, a
certain number by each party, so as to leave a number required by law to try
the cause. In general, a list of forty-eight jurors is made out for each
case; the plaintiff strikes off twelve, and the defendant the same number
from those who remain twelve are to be selected to try the cause, unless
they are challenged for cause. See Challenge.