Glasgow has always had a Catholic/Protestant divide, traditionally Celtic are associated with the Catholic side, Rangers with the Protestant.
Protestants hold in high regard King William of Orange. Therefore to a Celtic fan, they may be "Orange Bastards."(or basturts to use the local dialect).
Add the word "Dirty" to the beginning of the phrase and you have, "Dirty Orange Bastard" the initials of which form "DOB", which is where the widely-used Glasgow phrase "Dobber" originates from. Which, yes, means that if you're a dirty Rangers fan, and you call someone a dobber, you're slagging yourself. Up ye, and mon the hoops!
Check the nick of this dobber!