FOOLHARDY—venturesome, incautious, hasty, adventurous, rash. (Cautious.)
FOOLISH—simple, silly, irrational, brainless, imbecile, crazy, absurd, preposterous, ridiculous, nonsensical. (Wise, discreet.)
FOP—dandy, dude, beau, coxcomb, puppy, jackanapes. (Gentleman.)
FORBEAR—abstain, refrain, withhold.
FORCE, n.—strength, vigor, dint, might, energy, power, violence, army, host.
FORCE, v.—compel. (Persuade.)
FORECAST—forethought, foresight, premeditation, prognostication.
FOREGO—quit, relinquish, let go, waive.
FOREGOING—antecedent, anterior, preceding, previous, prior, former.
FORERUNNER—herald, harbinger, precursor.