Mammoth Uncensored Word List:
18 - 17 letter words starting with f

fantasticalnesses fashionablenesses feebleheartedness ferrimagnetically fibromyomectomies fibromyxosarcomas fickleheartedness fictionalisations fictionalizations fierceheartedness fissiparousnesses flirtatiousnesses floriferousnesses foresightednesses fractionalisation fractionalization fractoluminescent fragmentarinesses