Forming verbs and adjectives.
[Old English for-, faer-.]
This is no longer a living prefix, and many words formed with it are either obsolete (forwalk, forirk), archaic (fordo) or formal, poetic, or literary (forbear, forfend, forswear).
It was used to give increased force to a word (forlorn, pitifully sad and abandoned, from Old English lēosan, lose), indicate a prohibition (forbid), or variously suggest neglect, abstention or renunciation (forget, forgo, forsake, forgive).
Some words in for-, such as foreign, forest, and forfeit, derive from other Latin roots. Others appear to contain it through accidents of spelling: forage (Old French fuerre, straw) and forceps (the Latin word for tongs or pincers).