The Keyhole Turtleneck, often referred to as the 'chest keyhole' or 'oppai' sweater, became an internet phenomenon in late 2014 after a series of illustrations went viral on Japanese social media platforms like Pixiv and Twitter. The design features a high-neck sweater with a provocative cutout, blending traditional modesty with bold fanservice. It quickly evolved from a niche fashion illustration into a global meme, inspiring countless fan art pieces, cosplay photoshoots, and humorous comparisons. Today, it is used as a template to mock impractical fashion trends or to highlight the humorous contrast between conservative and revealing clothing styles.