Bones are the frame on which your body is built. They have joints at the elbows knees and shoulders. Some are hinge joints like knees and elbows and some are ball and socket, like shoulders and hips.
They are useful for working - that’s if you’re not lazy.
Idle means not doing anything; machinery can be idle and so can people.
In the war, it was that said idle talk cost lives. (By giving away secrets carelessly)
If you are bone idle, you are very lazy - maybe even wicked and lazy.