Safe and sound 
Completely save

Salad days
The awkward old time which could be happy to think of.

Salt of the Earth
People who dedicate their life to make the world better.

Salty dog
An expert sailor/ fisherman

Save face 
Save the reputation from public

Save your skin 
Stay away from problems

Sea legs
Do/ try something new.

Second to none
The best

See eye to eye
Two or more people are agree on something

See red
Very angry

Seed money
First amount of money to start your business.

Sell down the river
Betray. Not follow agreement.

Sell someone down the river
Betray someone

Shake a leg
Hurry up

Shoot off the mouth
Show opinion very loudly

Shot in the dark
Do something with little direction and hope to be successful.

Sit on the fence
Does not want to make the decision

Slip one's mind
Completely forget something

Smell a rat
Compete with someone

Snow job
Dishonesly talking

So to speak
Refer to someone's speech

Sooner or later
In the near future

Slip one's mind
Completely forget something

Speak of the devil
Someone you are talking about (probably in a negative way) is arrived.

Stand me up
Miss an appointment with me.

Stick out the neck
Take the risk