Frankpledge \Frank"pledge`\, n. [Frank free + pledge.]

(O. Eng. Law)
   (a) A pledge or surety for the good behavior of freemen, -- each freeman who was a member of an ancient decennary, tithing, or friborg, in England, being a pledge for the good conduct of the others, for the preservation of the public peace; a free surety.
   (b) The tithing itself. --Bouvier.

The servants of the crown were not, as now, bound in frankpledge for each other. --Macaulay.

Source: Webster's Revised Unabridged Dictionary (1913)