Great story.
OP deleted his original post that told the story: a guy named Mark Barnes apparently the coined the term while Manziel was a soph at Tivy. He had a big game filling in at QB, so Barnes went over to congratulate him, asked how to pronounce his name, then said, "I've just been calling you Johnny Football."
Teammates loved it, and it stuck.
Also a quote from Barnes saying he tends to make up nicknames for all the players there so he doesn't have to remember or pronounce last names. The kids there seem to love it.