Source : Webster's Revised Unabridged Dictionary (1913)

For- \For-\ [AS. for-; akin to D. & G. ver-, OHG. fir-, Icel.
   for-, Goth. fra-, cf. Skr. par[=a]- away, Gr. ? beside, and
   E. far, adj. Cf. {Fret} to rub.]
   A prefix to verbs, having usually the force of a negative or
   privative. It often implies also loss, detriment, or
   destruction, and sometimes it is intensive, meaning utterly,
   quite thoroughly, as in forbathe.