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.
2) Ligaments/Tendons. Ligaments and tendons are soft tissues that are positioned around all the joints in our body. Sometimes, when you move a joint (e.g. your knee), a ligament or tendon may stretch slightly as it goes over a small bony lump and then snaps back into place making a knee clicking sound.