| firm up | | | |
| first among equals | | | |
| first and last | | | |
| first annual | | | |
| first come, first served | | | |
| first loser | | | |
| first love | | | |
| first mile | | | |
| first-mover disadvantage | | | |
| first of all | | | |
| first off | | | |
| first of never | | | |
| first port of call | | | |
| first-rate | | | |
| first up | | | |
| fish-blooded | | | |
| fish-eating grin | | | |
| fishing expedition | | | |
| fish in troubled waters | | | |
| fish or cut bait | | | |
| fish out | | | |
| fish out of water | | | |
| fish to fry | | | |
| fist magnet | | | |
| fit for a king | | | |
| fit out | | | |
| fits and starts | | | |
| fit the bill | | | |
| fit to be tied | | | |
| fit to wake the dead | | | |
| five-star | | | |
| five will get you ten | | | |
| fix someone's wagon | | | |
| fix the roof while the sun is shining | | | |
| flag down | | | |
| flagellate | | | |
| flame up | | | |
| flannelled fool | | | |
| flap one's gums | | | |
| flash back | | | |
| flash for cash | | | |
| flash the cash | | | |
| flat chat | | | |
| flat on one's back | | | |
| flat out | | | |
| flat strap | | | |
| flatten out | | | |
| flavor of the week | | | |
| flea in one's ear | | | |
| flesh one's maiden sword | | | |
| flesh out | | | |
| flight of fancy | | | |
| flip-flop | | | |
| flip one's lid | | | |
| flip one's wig | | | |
| flip shit | | | |
| flip the bird | | | |
| float someone's boat | | | |
| flog a dead horse | | | |
| flog a dead pony | | | |
| flog the dolphin | | | |
| flog the log | | | |
| flood the zone | | | |
| floor it | | | |
| floor one could eat off | | | |
| floppy infant syndrome | | | |
| flower of the flock | | | |
| flunk out | | | |
| flush out | | | |
| flutter in the dovecote | | | |
| flutter the dovecote | | | |
| fly blind | | | |
| fly by | | | |
| fly-by-night | | | |
| fly by the seat of one's pants | | | |
| flying start | | | |
| flying visit | | | |
| fly in the face of | | | |
| fly in the ointment | | | |
| fly low | | | |
| fly off | | | |
| fly off at a tangent | | | |
| fly off the handle | | | |
| fly off the shelves | | | |
| fly on the wall | | | |
| fly out of the blocks | | | |
| fly out of the traps | | | |
| fly solo | | | |
| fly the flag | | | |
| fly too close to the sun | | | |
| fly under the radar | | | |
| fog a mirror | | | |
| fogged out | | | |
| fold like a cheap suit | | | |
| fold one's tent | | | |
| fold under zero pressure | | | |
| fold up | | | |
| folk devil | | | |
| follow someone off a cliff | | | |
| follow suit | | | |