Knee (noun)

1. The joint of the leg connecting the thigh and the lower leg, allowing for movement in the sagittal plane.
2. A part of a garment that covers the knee.
3. The part of a stair or step where one tread changes direction to meet the next.

Origin:

Old english cnēo, of germanic origin; related to dutch knie and german knie.

Examples:

1. He hurt his knee playing soccer.
2. She wore a dress with a knee-length hem.
3. The knee is a hinge joint.
4. He was on his knees praying.
5. The knee of the pants was worn out.