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.
There often is no significant problem with the joint or surrounding tissues. Or the noises can be associated with joint damage, inflammation or injury. The hip can make a clicking sound (or cause a sensation of clicking) for many reasons. For your age, the most common cause is likely the iliotibial band syndrome.