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.
1) Pain-free Popping: Popping noises in the knee often occur without any pain, in which case they are nothing to worry about. 2) Painful Popping Noise at time of Injury: Sometimes when the knee is injured e.g. twisting awkwardly, there is a sudden, loud pop at the same time indicating damage to part of the knee.