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.
If part of that valve balloons out so that it doesn't close properly, you have mitral valve prolapse. This causes a clicking sound as your heart beats. It's fairly common and it's often not serious. But it can lead to the blood flowing backward through the valve, also called regurgitation. Mitral valve or aortic stenosis: Your mitral or aortic valves are on the left side of your heart. If they narrow, which doctors call stenosis, your heart must work harder to pump blood out to the rest of your body.