It comes, of course, courtesy of actor / director and conservative Clint Eastwood, whose appearance at the Republican National Convention (RNC) was mocked on Twitter. It was also called "The weirdest thing I've ever seen at a political convention in my entire life" by Rachel Maddow (and if you've seen it, you might agree).
Eastwooding started as what's known as a "macro image meme," in which a contributor Photoshops and adds (sometimes) humorous captions to an image - in this case, of Eastwood and "the chair." This is what the “McKayla Maroney is not impressed” meme became. In fact, the two memes even crossed paths (above).
Check your local photo sharing social media service of choice: there are already thousands of images on Instagram, Tumblr and Twitter.