Meaning
A brown swirl of poop, usually drawn with a cheerful smiling face and big eyes. The cartoon look turns something gross into a joke, so it mostly signals silliness, toilet humor or calling something rubbish.
Friends use it to tease, to describe a terrible day or to rate something as bad: “My hair after the rain today 💩” It is rarely meant as a real insult, yet aiming it at someone’s work, food or photo can hurt, especially if they are not in on the joke. Keep it out of professional messages entirely.