The slang term Brum, referring to the city of Birmingham, England, has it’s origins in the twelfth century name of the town; Brummagem. Before this time the city was indeed named Bermingha, as chronicled by the Doomsday Book of 1087. This originates from the Anglo-Saxon where ham meant homestead, ingas meant of the people, and the area was named Beorma.
Due to the lack of compulsory education most people could not write, this meant that there was no official spelling for the town to refer back to and so the chroniclers and taxmen of the time would simply write down the name of the town as they heard it, which meant that the name slowly changed from Beormaingasham to Bermingha to Brummagem.
Unfortunately business in Brummagem was not always honest and so it became the case that the dictionary definition of Brummagem was “counterfeit.” Of course this made it inadvisable to advertise products as “made in Brummagem,” and local business man Mathew Boulton began to use the old “Birmingham,” as the name for his wares. This lead to Birmingham becoming the acceptable name for the town.
But Brummagem lived on as “Brum.” Because the change to Birmingham had been at a middleclass, educated level, the working class continued to use the old name for some decades, usually shortening it to the monosyllabic “Brum” because it was easier to say. The term Brummie meaning "to come from Birmingham" was born of this and also lives on to this day.