MRW is an abbreviation of 'My Reaction When...'. It is used to start a sentence, and does not work as a stand-alone phrase. It is usually accompanied by a .gif, video clip, or photo of an amusing reaction, usually taken from a popular movie or internet meme. MRW does not indicate a specific reaction on its own, but is most often used with visuals indicating disappointment, annoyance, mocking amusement, or anger.