Full and fat are synonyms for big. They begin with F.
fiendishfrenziedfrighteningfrenetic
FLY
* bluff * brief
Fennel is a vegetable. It begins with the letter f.
flaccid
flotilla
It means using profanity- specifically a word that begins with F that means sexual intercourse.
facts
formidable
flyover
Fog is a weather related word. It begins with the letter f.
FLU
justunfuf
fake
friendly
Forsooth
The answer is fail.