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.
This condition causes pain around or behind the kneecap, and can feel worse when walking or running downhill or descending stairs; you may even hear a crunching or clicking sound when you bend or extend your knee. There are several exercises you can do on a daily basis to remedy the problem.