Bending the Elbow Rule

Even the crudest among us has heard you’re not supposed to put your elbows on the table. In fact, it’s so familiar, there’s even a nursery rhyme about it—and you know you’ve arrived when you have your own nursery rhyme, just like the plague (Ring Around the Rosie) or a homicidal English queen (Mary, Mary, Quite Contrary). It goes:

Mabel, Mabel, strong and able
Get your elbows off the table
This is not a horse's stable

But why?