Here is our list of American idioms that start with "S":

Safe and sound:
Safe. Unharmed. Example: "It was a rough trip but we got there safe and sound."

(To) save money for a rainy day:
To reserve something for some future need. Example: "I've saved a little money for a rainy day."

(To) save the day:
To produce good results when bad results are expected. Example: "Colin saved the day by remembering to bring water."

School (someone) (v.): To teach someone a lesson. To show someone how something is (really) done. Example: "We thought we were the best, but the other team really schooled us."

(A) score to settle:
To get even. To pay someone back for something negative that they did. Example: "Don't stop me. I have a score to settle with him."

(To) scratch the surface:
To begin finding out about something. Example: "We've only begun to scratch the surface in this field."

Screw up (v.): To make a mistake; to do something badly or wrong. Example: "I really screwed up my audition."

Second nature (to someone):
Easy and natural. Example: "Scoring goals is second nature to him."

(To) see eye to eye on something:
To have a similar opinion on something. Example: "Despite their public differences, the two candidates actually see eye to eye on most issues."

(To) see fit:
To deem/believe to be appropriate. Example: "You can do that any way you see fit."

(To) see the writing on the wall:
To see that something is going to happen.

(To) serve someone right:
To serve as appropriate punishment for someone. Example: "They put him in jail for 5 years? Serves him right!"

(To) set foot somewhere:
To go or enter somewhere. Example: "If I were you, I wouldn't set foot in that town."

(To) set one's sights on something:
To select something as one's goal. Example: "We would like to buy out one of our competitors. We've got our sights set on Sony."

Sharp (adj.): When talking about people = smart, intelligent; when talking about clothes = stylish, high-quality.

Shoot hoops (v.): To play basketball in a casual, informal way. Example: "Let's shoot some hoops on Friday."

(To be) short on cash: To not have too much money. Example: "I'm a little short on cash. Could you lend me some money until Tuesday?"

(A) shot in the arm:
A boost. Example: "The good financial news was a real shot in the arm for Steve's company."

Show, Show up (v.): To arrive. Example: "We waited for two hours, but they didn't show (up)."

(To) sit on one's hands
To do nothing. Example: "Everyone expected the board to take action, but they just sat on their hands."

(To) sit tight:
To wait (patiently). Example: "Sit tight, your mother will be here soon."

Skeleton(s) in the closet:
A shocking/ disturbing secret.

(The) slammer (n.): Jail. Example: "He was in the slammer."

(To) sleep on something:
To think about something overnight. Example: "I'm not sure if I want to buy this car. I think I should sleep on it."

Snail mail (n.): Regular physical mail (as opposed to email).

Something (n.): In phrases such as "Wasn't that something!", "something" actually means "something interesting" or "something special" or "something surprising", depending on the context.

Space-case/Space-cadet (n.): Very strange person who always seems to be in his/her own world. Example: "Peter is a bit of a space cadet. He's always zoning out when I talk to him."

Space out (v.): To lose oneself in thought, esp. when you should be paying attention to something else. To zone out. Example: "Sorry, I spaced out for a second. What were you saying?"

Split (v.): To leave. Example: "I have to split. See you tomorrow!"

Split-up (v.): To break up; to stop seeing each other (as boyfriend/girlfriend, husband/wife, etc.) Example: "Tom and Maria split up last month."

Spook (v.): To scare.

Spread (n.): Collection. Example: "They had quite a spread of food at the party."

(To) stand up for something:
To fight for, support. Example: "The elected official promised to stand up for the poor."

Stash (n.): A hidden collection of something, often (but not always) used to refer to hidden drugs. Example: "Mark's parents found his stash (of pot) under his bed."

Stick to (something): To keep (doing something). To not stop (doing something) Example: "Cynthia wants to be an actress, but I think she should stick to singing."

(To) stir up trouble:
To cause trouble. Example: "Sometimes I think she gets great pleasure from stirring up trouble."

Straight (adj.): 1) Direct, clear, honest. Example: "He wouldn't give me a straight answer."; 2) Heterosexual. Example: "I'm pretty sure he's straight."

Straight up: Honestly/Honest.

Strike up a conversation: To start a conversation. Example: "He walked up to the first woman he saw and struck up a conversation with her."

Stuck up (adj.): Snobbish, conceited. Example: "I don't like her. She's so stuck up."

Stunner (n.): Attractive woman. Example: "Wow! What a stunner!"

Sucker:
A gullible person or someone who is easily impressed by something (eg. "a sucker for gadgets" = someone who is easily impressed by gadgets)

Sugar daddy/sugar mama(n.): A wealthy man or woman who either supports you financially (pays for you) or gives you gifts, in return for sex or companionship.

Suit yourself:
Have it your way; As you wish; Example: "I wouldn't walk around that neighborhood at night, but if you really want to, suit yourself."

(To) swallow something hook, line, and sinker:
To believe something (usually a lie) completely. Example: "Margaret told him a flat-out ( = complete and utter) lie and he swallowed it hook line and sinker."

Sweet (adj.): Very nice. Example: "That is a sweet car."

(To have a) sweet tooth: To love to eat candy, sweets, etc.

Swig (n.): A drink (out of a bottle). Example: "Gimme a swig!"

Swing (something) (v.): To arrange (something)/to get (something) done. Example: "It'll be hard, but we'll probably be able to swing a deal."