It is unclear how or why the jackets were made - they were merchandise for an athlete who was injured horribly during the most important match of his career. Others say it was from a line of clothes that catered to jocks, and the owner brought in some sort of spellcaster to place a spell on the brand so as to ensure that he had a healthy supply of his demographic. And there is also the theory that it was created by some sort of deity of fertility or health - whatever the case, rumours of these jackets circulate on the web.
A nameless Letter jacket that over time transforms its wearer into either a high school jock, a college jock, or professional sports player. It was sought out by many, be they athletes searching for improvement, or outcasts looking for a way into popularity.
Schools and their administration do not seem to notice the odd effects, and accept the new jocks as if they were always like that. Even the parents seem to be affected in this way. Some of these young men took to it well, while others took the world changing around them markedly less well.
What is more, over time, the wearer's mind becomes fitted to his body, as if the jacket was acting even on his mind.
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So now, you stand in front of the school, looking down at a jacket resting, perfectly folded up, on a bench. It looks suspiciously clean for being out here, on a public bench. First of all, of course, you shout out to see if this jacket belongs to anyone. No response. Nobody seems to know whose it is. So you decide to take it with you, at least until you find its owner. Or maybe you decide that it is a pretty cool jacket, so it could be a keeper.