Klutz

A 'klutz' is a clumsy person who trips a lot and/or drops things.

Example:
-Dad is such a klutz!
-Yeah, he broke Mom's favorite vase!

Knock it off!

'Knock it off' is an informal way someone tells you to stop doing something. The person who says it has more power than you do. It's often said by a parent, or in an informal context.

Example:
-Hey you guys, for the last time, knock it off!
-Sorry, Dad!

Kickstand

A 'kickstand' is the little metal bar that you push down to the ground with your foot when you park your bike. It keeps your bike from falling down.

Example:
-Don't forget to use your kickstand so your bike doesn't fall down.
-I will, Mom.

Keeping tabs on

If someone is 'keeping tabs on' you, it means they are paying attention to everything you do, as if they want to know your every move. It's very frustrating if someone is keeping tabs on you.

Example:
-Your parents are constantly asking me where I'm going or where I've been. It's driving me crazy.
-I'm really sorry they're keeping tabs on you. I'll talk to them.

Knee-jerk reaction

A 'knee-jerk reaction' is the reaction you have to something without giving it much or any thought.

Example:
-Why did you swerve into the other lane of traffic?
-I had a knee-jerk reaction. I saw the deer and didn't want to hit it!

Knee deep in

'Knee deep in' is used to describe when someone is in the middle of doing something and doesn't want to or can't be interrupted. The visual it suggests is that the person is standing in something that is up to their knees, and they can't easily get out, if out at all. It can be used as an apology.

Example:
-I'm sorry I can't go to lunch with you! I'm knee-deep in paperwork!
-Maybe tomorrow!