This is a little like the older gentleman we saw earlier wearing the 'YOLO' t-shirt, only taken to the next level. When we reach a certain age, society expects us to start dressing more conservatively. We're supposed to throw away the t-shirts, and perhaps get a nice sweater or cardigan to keep us warm. Shirts with slogans on them are the sort of thing this man's grandchildren should be wearing. Maybe he's even borrowed it from one of them!
We actually like his choice of style, though. Not caring about other people's opinions of us is a trait that's equally desirable at any stage of life, and it even doubles up as a message to the people who think he should be wearing more age-appropriate clothing. Got a problem with what he's wearing? Well, wussup haters?