Hiya Primero
A 'Brummie' is what you call a person who comes from Birmingham, England's second largest city.
Some people call Birmingham, Brumijum (I have spelled it the way you say it!) which I think is what gave them the name, 'Brummie'.
Don't worry, it is only swearing when, like Nobby, YOU CALL IT SOMONE WHO
ISN'T A BRUMMIE

I come from an area which is about 10 miles from Birmingham which comes within a region called the 'Black Country'.
It got it's name because the coal burnt during the industrial revolution.
(The story goes that Queen Victoria would close the curtains of her carraige when she passed through the area)
To outsiders (usually Brummies!) we are called Yam-yams.
I hope this helps.