That term gets misused pretty often to describe any sort of bearing failure. When it actually happens, a bearing such as a connecting rod or main gets worn thin enough that the ends of the bearing overlap, one bearing slides under the other, and wedges itself tight enough that the crankshaft usually locks up. If this happens to a connecting rod, the rod will sometimes break and the broken-off piece spins around inside the block. That brings on another misused term- - - -"thrown rod". That usually ends up with holes in the block and/or the oil pan. Occasionally, an engine will keep running if the spun bearings are worn badly enough that they don't wedge together. The knocking noises from a bearing that bad are horrendous! By the time a bearing gets loose enough to knock heavily or spin, the fix usually involves installing another crankshaft, and replacing or reconditioning all the connecting rods.
When an engine starts losing oil pressure and making lower-end noises, some people do their best to ignore them, hoping the problem will go away. That's like being just a little bit pregnant!
Jerry
