Kangaroo court
Unauthorized court / unofficial court.

Keep at bay
Keep a safe distance.

Keep a low profile
Maintain self-privacy.

Keep in touch        
Continue to be in contact

Keep mum
Keep quiet.

Keep one's ear to the ground
Focus on something to find out what is going on.

Keep one's feet to the ground
Be realistic. Accept the current situation.

Keep posted        
Keep updated

Keep the finger cross
Hope bad things will not happen

Keep the chin up
Be brave / don't be discourage

Keep the shirt on
Calm down

Keep it for a rainy day
Keep something for the future when it might be needed

Keep the ball rolling
Continue doing something

Kick body and soul together
Work just enough to stay alive.

Kick away someone's the ladder
Eliminate someone's support

Kick the bucket
Die

Kick up someone's heel
Celebrate with someone

Keep your eye on the ball        
Keep focus on something

Keep your nose clean        
Stay away from any trouble

Kick the der        
Take out something that support them

Kill to birds with one stone 
Do two things at the same time.