'Unbelievable' was a number 1 hit single for EMF in 1990 when it was the 32nd best selling British single that year. It featured dialogue samples from Andrew Dice Clay and has a very rude word in the chorus, if you know where to look.
Jeff is short for Jeffery, which means 'God's piece' or 'divine piece'.
Put the two together and you get UNBELIEVABLE JEFF, a super cool super hero often seen flying through the sky over football (that's 'soccer' to our transatlantic viewers) matches every Saturday afternoon wearing a retro t-shirt that bears his his name. With the atmosphere being so thick and fast they'd be fighting like beavers if he wasn't there to keep an eye on them. Not only would he have to show them the red card, he'd have to send them off as well. After all, they only count when they go in the goal.