The Collaborative International Dictionary of English v.0.48:

Friborg \Fri"borg\, Friborgh \Fri"borgh\, n. [AS. fri[eth]borh,
   lit., peace pledge; fri[eth] peace + borh, borg, pledge, akin
   to E. borrow. The first part of the word was confused with
   free, the last part, with borough.] (Old Eng. Law)
   The pledge and tithing, afterwards called by the Normans
   frankpledge. See Frankpledge. [Written also friburgh
   and fribourg.] --Burril.
   [1913 Webster]