I always wondered about this one. I guess I should have figured it out for myselft but things are always much clearer when someone spells it out for you…
Spinning Beach Ball of Death (SBBOD)
Officially the “spinning wait cursor” the spinning beach ball of death spells trouble to most Mac users, so we usually just curse at it. Making its first appearance in OS X, it tells us an application is not responding, so we either have to force quit it, or wait.
The ball, as many of us know, was originally a wrist watch in Mac OS 9. The rainbow ball is a natural extension, but where did it come from? It was, in the shape of an optical disk, used in NeXTstep, a precursor to OS X. The ball was supposed to represent the removable and rewritable magneto-optical disks NEXT computers ran from. But, regardless of where it came from, we still groan when we see it.
via The Mysterious Origins of the Command Key and the Beach Ball | Mac|Life.