Facetious :---> joking, humorous.
Facilitate :---> help bring about.
Fallacious :---> false, misleading.
Fallible :---> liable to error.
Fatuous :---> foolish, inane.
Fawning :---> courting favor by cringing.
Feign :---> fake, imitate, pretend.
Felicitous :---> apt, suitably expressed.
Fervor :---> growing ardor
Fidelity :---> faithfulness, authenticity
Fledgling :---> inexperienced.
Florid :---> ornate, showy, Gaudy.
Flout :---> reject, mock.
Fluctuate/ fluctuation :---> wavering.
Foment :---> stir up, instigate.
Foray :---> raid, make excursion.
Forestall :---> prevent by taking action in advance.
Frugality :---> Economy.
Furtive :---> stealthy, secret, surreptitious.