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.
Chondromalcia patella (patellofemoral pain syndrome) is a dull ache behind the kneecap (patella), usually caused by overuse or injury. You can feel and hear a painful crunching and grating when the knee is moved. Other knee injuries or types of arthritis can also cause crepitus.