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.
Cavitation is a common term used by chiropractors and the cracking sound you may hear when having a chiropractic adjustment. It is a result of a a force causing a temporary vacuum within the joint, which then collapses and causes a clicking noise. Gas bubbles within any joint fluid can also cause this snapping noise.