'Hun' applied to Rangers fans is the same word as the British used to describe Germans in the World Wars. Basically, it means a 'barbarian' or 'savage', based on the ancient race who overran Europe under Attila the Hun in the 5th century. Obviously, Celtic fans think their rivals are primitive savages!
Quizmonster is quite correct, although both sides have their 'Huns' particularly when they lose to each other!!! In recent weeks both Celtic fans and players have been involved in controversy. Jock Stein (God bless him) must be spinning in his grave at the antics these days.