Sit through
If you sit through something like a long speech or a boring show, you wait until it's over before leaving, even though you're not enjoying it.
sit up (1)
If you sit up, you get up into a sitting position after you've been lying down.
sit up (2) 
to not go to bed until later than usual
Size up
to observe carefully and then form an opinion about a person or a situation
Sleep together INFORMAL
to have sex together
Sleep with INFORMAL
to have sex with someone
Slip up INFORMAL
to make a small mistake, usually because of carelessness
Slow down
to become slower, or to make someone or something go slower
Smell of
to have a particular smell
Snap up
to get something quickly so you don't miss out
Sober up
to become sober again after drinking too much alcohol
sort out (1)
If you sort things out, you arrange them into some sort of order.
sort out (2)
If you sort something out, you make arrangements for it or you organize it.
Speak out
If you speak out, you publicly state your position on an issue, or publicly oppose or defend someone or something.
speak up (1)
to speak louder
speak up (2)
If you speak up, you publicly state your position on an issue, or publicly oppose or defend someone or something.
Speed up
to move faster, or to increase the speed of something
Spell out
If you spell something out, you explain it slowly and very clearly so that everyone can understand.
Spread out
If a group of people spread out, they move apart so that there's more space between them.