Spud is correct... it's the flour made of grinding the entire wheat kernel... think you Brits call it whole meal flour... same flour used to make graham crackers. However, the originator of the flour was an American nutritionist in the early 1800's with some unusual ideas... Word Origins has this to say: Sylvester Graham (1794-1851), by the standards of his day, was a crackpot. In the early 1800s, when Americans believed good eating meant as much meat and fowl as one could consume, the fatter the better, Sylvester preached a diet that would do a 1990s nutritionist proud. The base of his food pyramid was whole wheat flour, coarsely ground and not sifted. He would allow bread made of this flour, provided the bread was not too fresh; it had to stand for at least twelve hours after baking. Graham's menu also included other grains, likewise coarsely ground, and vegetables and fresh fruits. To drink? Water, of course.
Grahamism and Grahamites flourished in the 1830s in the wake of popular lectures by Graham, a minister who was general agent for the Pennsylvania Temperance Society. Aside from the plain diet, Graham also, according to the Dictionary of American Biography, "recommended hard mattresses, open bedroom windows, cold shower baths, looser and lighter clothing...and cheerfulness at meals."