Today's Christmas edition of Great Truths is how meanings change over time.
When I was a kid the threat was, if you're naughty, Santa will bring you a lump of coal. Of course to us this meant something dirty and useless. They might as well have said Santa would take a dump in your stocking. BUT...
If you were born earlier--we'll say WWII or before--a lump of coal was actually something very useful to get. Coal could be burned in the stove to heat the house and cook the food to keep the family from freezing and starving to death. Still not anything fun for a kid to play with, but it was a message to the "naughty" kid about giving and sharing. You shared your present from Santa with your family so you would all make it through the cold winter together. And maybe next Christmas, if you were nice, you might get candy or a toy or something.