I don't think that is a shirt, but rather a cheep and jauntily worn (aka Michael Jackson) trilby hat... come to think of it... the clothes would probable fit Michael... who really was a "slender man."
Hmmm... this could lead to a number of rather provocative theories that would explain the intersection between title and illustration...
I'm afraid I'd be to embarrassed it elucidate any further... ^_^
