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.
Approximately 50% of ACL tears are accompanied by a popping sound associated with immediate swelling and pain. The other classic sign of an ACL tear is the knee repeatedly giving way. The knee pain and popping only occurs at the imte of the injury, there isn't usually any recurring noises with knee movement.