Welcome to some British slang.
Smart - When we say someone is smart, we are talking about the way they are dressed - you might say they look sharp. When you say someone is smart you are talking about how intelligent or clever they are.
Smashing - If something is smashing, it means it is terrific.
Smeg - This is a rather disgusting word, popularised by the TV show, Red Dwarf. Short for smegma, the dictionary definition says it is a "sebaceous secretion from under the foreskin". Now you know why it has taken me 3 years to add it in here. Not nice! Rather worryingly smeg is also the name of a company that makes ovens!!!
Snap - This is the name of a card game where the players turn cards at the same time and shout "snap" when they match. People also say "snap" when something someone else says has happened to them too. For example when I told somebody that my wallet was stolen on holiday, they said "snap", meaning that theirs had too!
Snog - If you are out on the pull you will know you are succeeding if you end up snogging someone of the opposite sex (or same sex for that matter!). It would probably be referred to as making out in American, or serious kissing!
Snookered - If you are snookered it means you are up the famous creek without a paddle. It comes from the game of snooker where you are unable to hit the ball because the shot is blocked by your opponent's ball.
Sod - This word has many uses. My father always used to say "Oh Sod!" or "Sod it!" if something went wrong and he didn't want to swear too badly in front of the children. If someone is a sod or an "old sod" then it means they are a bit of a bastard or an old git. "Sod off" is like saying "piss off" or "get lost" & "sod you" means something like "f*** off". It also means a chunk of lawn of course. You can usually tell the difference!
Sod all - If you are a waiter in America and you serve a family of Brits, the tip is likely to be sod all or as you would call it - nothing. Because we don't know about tipping.
Sod's law - This is another name for Murphy's law - whatever can go wrong, will go wrong.
Sorted - When you have fixed a problem and someone asks how it is going you might say "sorted". It's also popular these days to say "get it sorted" when you are telling someone to get on with the job.
Speciality - This is another one where you chaps drop your "I". when I first saw specialty written down in the US I thought it was a mistake. But no! We love our I's!
Spend a penny - To spend a penny is to go to the bathroom. It is a very old fashioned expression that still exists today. It comes from the fact that in ladies loos you used to operate the door by inserting an old penny.
Splash out - If you splash out on something - it means you throw your senses out the window, get out your credit card and spend far too much money. You might splash out on a new car or even on a good meal.
Squidgy - A chocolate cream cake would be squidgey. It means to be soft and, well, squidgey!
Squiffy - This means you are feeling a little drunk. Some people also use it to mean that something has gone wrong.
Starkers - Avoid being seen starkers when visiting England. It means stark naked.
Stone the crows - This is an old expression with the same meaning as "cor blimey".
Stonker - This means something is huge. Looking at the burger you might say "blimey what a stonker". It is also used to refer to an erection! Clearly English modesty is a myth!
Stonking - This weird word means huge. You might say "what a stonking great burger" if you were in an American burger joint.
Strop - If someone is sulking or being particularly miserable you would say they are being stroppy or that they have a strop on. I heard an old man on the train tell his wife to stop being a stroppy cow.
Clancy's comment: Mm ... Some of these are also used in Australia. The nice ones.