In 2015 it was the oversize pool float, which appeared on Instagram in unicorn or flamingo form, endlessly squished between moist, golden thighs. In 2017 it was the RompHim, a carefree merger of shorts and shirt that started as a Kickstarter campaign and somehow swelled to seemingly capture all of global douchedom in a single pastel garment. In 2019 you’d be wise to prepare your eyeballs for this year’s version—the beach blazer. It’s a half-tuxedo, half-towel concoction that solves the eternal summer conundrum: How do I dress formally, but also not wear pants? This is what Manhattan web developer Marko Andrus was daydreaming about in early 2014, as he looked toward a season in the Hamptons. “What’s that cozy layer you bring to the beach and put on at the end of the day?” he wondered. “For many people it’s a hooded sweatshirt, which I think is kind of gross. You’re not making a chic fashion statement.” Andrus ordered some yards of terry cloth and took them to his tailor on the Upper East Side to be made into a blazer. A few weeks later, “I was running around on Nantucket in it,” he recalls. “People were stopping me on the beach and saying, ‘Who the hell are you, and where did yo...