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

Keel over (v.): A rude way of saying "to die".

(To be) keen on: To like; To be fond of; Example: "I'm not really keen on this idea."

(To) keep a low profile:
To lay low; to not attract attention to oneself by talking loudly, dressing in flashy clothes, causing trouble, etc. Example: "The bank robbers knew that they had to keep a low profile until they crossed the border."

(To) keep an eye on (someone/something):
To watch/pay attention to (someone/something). Example: "I have to go to the store. Can you please keep an eye on the baby for 10 minutes?"

(To) keep a straight face:
To force oneself not to laugh, even though one wants to. Example: "He was saying the stupidest things, and I was finding it hard to keep a straight face."

(To) keep one's cool:
To remain calm. Example: "It's not easy to keep one's cool in stressful situations."

(To) keep one's word:
To uphold one's promise. Example: "He's a very honorable person who always keeps his word."

(To) keep someone posted:
To keep someone informed. Example: "I'm not sure what the plan is for this evening, but I'll keep you posted."

(To) keep someone up:
To not allow someone to sleep. Example: "The neighbors' party kept me up all night."

(To) keep something to oneself:
To not let others know about something; to not reveal something. Example: "Hey, don't tell anyone about your time in prison. Keep it to yourself."

(To) keep track of (something):
To maintain a record of (something). Example: "I don't know where all my money goes. It's hard to keep track of my expenses."

(A) kept man/woman:
Someone who is in a relationship where the other person pays for everything. Example: "Stephen is a kept man. His girlfriend always pays for everything."

(To) kick back:
To relax. Example: "My brother and I are going to kick back and have a couple of beers."

(To) kick oneself:
To regret. Example: "I'm kicking myself for not having bought that car when I had the chance."

(To) kill time:
To waste time. Example: "I'm just killing time until my brother arrives."

Killer (adj.): Great; amazing. Example: "A killer new application", "A killer new product"

Klutz (n.): A clumsy person. Example: "My brother is such a klutz. He's always bumping into things and knocking them over."

Knock (v.): To talk negatively/badly of; to criticize, badmouth. Example: "Don't knock it until you try it/ until you've tried it."

Knock it off!:
Stop it!

Knock on wood:
A phrase said to cancel out (imaginary) bad luck. *Touch wood* is also used in some parts of the English-speaking world.

Knock (someone) up: To get (someone) pregnant; Knocked up = pregnant.

(To) know something inside-out:
To know something completely and thoroughly. Example: "Let me show you around. I know this neighborhood inside-out."

Know-it-all (noun):
A person who acts like they know everything (slightly derogatory). Example: "P1: Robert is so conceited! P2: Yeah, he's a real know-it-all."