Face the music
Accept the negative consequences of something

Face to face
Seeing each other (usually use in a meeting).

Face value
Interest in appearance more than the detail inside

Fair and square
Very fair

Fall between two stools
Something try to be two things but is not good at being either

Fancy man
new lover

Fat cat
Very rich person

Fat head
Stupid person

Feed someone a line
trick someone

Feel blue
Feel sad

Feel like a million dollar
Feel very good

Feeling a bit under the weather
Feeling a bit sick

Fishy smell            
Something strange

Flea market            
A Market where sells second hand goods

Flood the market
Too much supply (products or services) in the market.

Fly a kite            
Test reaction of the idea

Fly blind            
Work without training or instruction

Follow one's nose
Strictly follow one's direction.

Food for thought
Something to think about

For ages
Very long time

For the birds            
Something which are not interesting

Fork over
Give / offer