fragile, frail, brittle, and delicate
freedom, familiarity, liberty, and independence
frightful, fearful, direful, and dreadful
frivolous, trifling, petty, and childish
fruitful, fertile, prolific, and productive
fruitless, vain, trivial, and foolish
frustrate, defeat, disappoint, and thwart
fully, completely, abundantly, and perfectly
furious, impetuous, boisterous, and vehement