1727, from Japanese samurai warrior, knight, originally the military retainer of the daimio, variant of saburai, nominal form of sabura(h)u to be in attendance, to serve..
The bubbles make a noise when they burst. This is called cavitation. Another cause for painless popping in the knee is when the ligaments and tendons catch as they go over a bony lump and pop when they snap back into place. Or the clicking and popping may be caused by catching on soft tissue or scar tissue within the knee.
The movement of the kneecap is always visible and, often, intensely painful. Iliotibial band syndrome. This syndrome occurs when a band of tissue rubs against the outer portion of your femur (thigh bone), causing sharp, burning pain on the outer side of the knee. Although this can result from a direct injury to the knee, often the cause is the stress of long-term use, such as long-distance running. Plica syndrome.