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.
Gout, which is an abnormal buildup of uric acid in a joint, is another type of arthritis that can cause the knee to swell. X-rays, MRI's and blood tests can help to detect if arthritis is the cause of fluid on the knee and determine which type of arthritis is present.