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.
The IT band is often the hidden cause of many people's outer knee or hip pain. Iliotibial band friction syndrome can cause a nagging, dull ache, or flare into a sharp, acute pain felt at a specific spot on the outside (lateral aspect) of the knee and lower thigh. For some athletes, a tight it band may cause hip pain as well.