- Kangaroo court
- Katy bar the door
- Keel over
- Keen as mustard
- Keep a stiff upper lip
- Keep a wide berth
- Keep at bay
- Keep body and soul together
- Keep it under your hat
- Keep shtum
- Keep the ball rolling
- Keep up with the Joneses
- Keep your chin up
- Keep your hands clean
- Keep your distance
- Keep your nose clean
- Keep your nose to the grindstone
- Keep your pecker up
- Keep your powder dry
- Keeping up with the Joneses
- Keepy-uppy
- Kettle of fish
- Khyber pass
- Kick the bucket
- Kick your heels
- Kickin'
- Kilroy was here
- King James' Version of the Bible
- King's ransom
- Kiss and tell
- Kiss me Hardy
- kit and caboodle
- Kith and kin
- Knee-jerk reaction
- Knick-knack
- Knight in shining armour
- Knock back
- Knock into a cocked hat
- Knock off
- Knock on wood
- Knocked for six
- Know the ropes
- Know your onions
- Know which side your bread is buttered
- Know which way the wind blows
- Knuckle down
- Knuckle under
- Kowtow to