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.
Knee pain and popping is a common problem and can be caused by a number of factors. Another term commonly used for popping in the knee is Crepitus, which essentially means a noisy joint, whether it be popping, cracking or snapping. Popping noises in the knee usually fall into one of three categories: