keep a low profile: try not to be noticed by others; avoid drawing attention to oneself.

Darryl decided to keep a low profile when he first joined the company and spent a lot of time in his cubicle.

keep an eye on (something or someone): watch to prevent theft of a thing or injury to a person.

You have to keep an eye on your kids while they're swimming.

I try to keep an eye on the news in order to make financial decisions.

keep a straight face: try not to smile or laugh.

It's hard to keep a straight face when in the presence of clowns; however, many children find them to be kind of scary.

keep (someone) company: to spend time with someone; to be with another person.

Rhonda knows that her mother is lonely after the death of her father, so she keeps her mother company on the weekends.

keep your eye on the ball: try to achieve your goal; do what you have to do to.

If you don't keep your eye on the ball, you're not going to be successful.

keep your mouth shut: to be quiet and not say anything.

Bobby has to learn how to keep his mouth shut because he says a lot of things that hurt other people's feelings.

keep to oneself: to isolate oneself from other people; to avoid communicating with others; to have private feelings and thoughts.

Unlike other people in our class, Mary keeps to herself and doesn't talk to the other students.

keep up with the Joneses: to have things that your neighbors have; not to fall behind in acquiring material wealth.

It's hard to keep up with the Joneses when you're out of a job.

kick the bucket: die

Arnold was living a very good life until he kicked the bucket.

(caution: this is a very informal expression and some people might take offense in the use of it.)

kill two birds with one stone: to achieve two goals with one action.

Growing vegetables provides fresh food and is an interesting hobby thereby killing two birds with one stone.

knight in shining armor: a person who provides help to another person, usually a man who helps a woman. This is a reference to the days of medieval chivalry.

Although not everyone likes Brett because he's not a very nice person, he's Mary's knight in shining armor.

knock it off: stop it; quit doing that. (This is a command similar to "Stop it" and "Quit it."

Emily and Taylor were fighting with each other until their father told them to knock it off.

knock one's socks off: to make a good impression on someone; to like something or someone very much.

The Thai food at that restaurant will knock your socks off.

knock the living daylights out of (someone): to hit someone very hard.

Dick "The Bruiser" Kelly knocked the living daylights out of his opponent in last night's fight.

know better: to understand the difference between right and wrong.

Jimmy likes to eat sand but that's only because he doesn't know better.

know one's place: to understand how you fit within an organization; to know who you are in a social setting.

Knowing your place within a company will make it easier to get along with coworkers.

know one's stuff: to know your subject well; to be professional and knowledgeable.

Professor Jenkins really knows his stuff when it comes to computers and the internet.

know the ropes: to know how to do a particular job. (similar to "learn the ropes.")

Until you know the ropes around here, you're going to have to spend extra in getting the right information from other people.

know where one stands: to understand one's relationship with other people or within an organization.

James thinks he knows where he stands with Crystal, but he's not completely sure how much she likes him.