Definition of Firdwite
In old English law, a fine for refusing military service (mulcta detrectantis militiam). A mulct or penalty imposed on military tenants for their default in not appearing in arms or coming to an expedition.
A fine imposed for murder committed in the army; an acquittance of such fine.
That's the definition of Firdwitein Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.