Wiki says he lost it in a meat cutting machine while working at a fast food place during his high school years.
That is only half-true. The wiki author forgot the stupidity/stubbornness part. He cut his finger badly, but not off. Instead of going to the hospital to get the wound cleaned and stitched up, he decided to go for a swim in Lake Michigan. He ended up with a particularly nasty infection that nearly killed him, so it had to be amputated.
My personal favorite answer would be that he tried to pet Barney.
