I will be very angry at you if you do.

"I'm going to tell Billy what you said about him." "Don't you dare! "

• a fifty-fifty chance
• a fight for life
• a fighting chance
• a fig leaf
• a figment of imagination
• a figment of your imagination
• a figure of fun
• a fine kettle of fish
• a fine line
• a finger in every pie
• a firm hand on the tiller
• a fishing expedition
• a fish out of water
• a flash in the pan
• a flight of fancy
• a flight of fantasy
• a flight of imagination
• a flip-flop
• a fly in the ointment
• a fly on the wall
• a fool and his money are soon parted.
• a fool's errand
• a force to be reckoned with
• a foregone conclusion
• a four-letter word