The Collaborative International Dictionary of English v.0.48:

Wapentake \Wap"en*take\ (?; 277), n. [AS. w?penge??c,
   w?pent[=a]c, from Icel. v[=a]pnat[=a]k, literally, a weapon
   taking or weapon touching, hence an expression of assent ("si
   displicuit sententia fremitu aspernantur; sin placuit frameas
   concutiunt." --Tacitus, "Germania," xi.). See Weapon, and
   Take. This name had its origin in a custom of touching
   lances or spears when the hundreder, or chief, entered on his
   office. "Cum quis accipiebat praefecturam wapentachii, die
   statuto in loco ubi consueverant congregari, omnes majores
   natu contra eum conveniebant, et descendente eo de equo suo,
   omnes assurgebant ei. Ipse vero, erecta lancea sua, ab
   omnibus secundum morem f[oe]dus accipiebat; omnes enim
   quot-quot venissent cum lanceis suis ipsius hastam tangebant,
   et ita se confirmabant per contactum armorum, pace palam
   concessa. Waepnu enim arma sonat; tac, tactus est -- hac de
   causa totus ille conventus dicitur Wapentac, eo quod per
   tactum armorum suorum ad invicem conf[oe]derati sunt." --L L.
   Edward Confessor, 33. D. Wilkins.]
   In some northern counties of England, a division, or
   district, answering to the hundred in other counties.
   Yorkshire, Lincolnshire, and Nottinghamshire are divided into
   wapentakes, instead of hundreds. [Written also wapentac.]
   --Selden. Blackstone.
   [1913 Webster]

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

WAPENTAKE. An ancient word used in England as synonymous with hundred.
(q.v.) Fortesc. De Laud. ch. 24.