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!