When I used to first work with teenagers, they started calling me Triple XL because I wore an extra large T-shirt. I joked that I was XXXL because I was so "Phat" or cool! They then started calling me Phatboy, with a wink and a nudge that the name was spelled P H A T instead of F A T!
So the name has stuck, although now they call me Pheezo Beezo!