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.
Chondromalacia patella refers to the softening and breakdown of the cartilage on the underside of the kneecap or patella. It is one of the common causes of knee pain, especially in women. Chondromalacia patella is caused by misalignment, or improper tracking, of the patella as it slides over the thighbone (femur). The most common symptom is a dull, aching pain in the front of the knee behind the kneecap, which can be exacerbated by prolonged sitting. Potential causes of the condition include muscle imbalances such as tightness in the iliotibial band and weakness in the quadriceps or hip rotator muscles.