for-

CIDE DICTIONARY

for-,  [AS. for-; akin to D. & G. ver-, OHG. fir-, Icel. for-, Goth. fra-, cf. Skr. parā- 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.  [1913 Webster]

OXFORD DICTIONARY

for-, prefix forming verbs and their derivatives meaning:
1 away, off, apart (forget; forgive).
2 prohibition (forbid).
3 abstention or neglect (forgo; forsake).
4 excess or intensity (forlorn).

Etymology

OE for-, f{aelig}r-