The Rice University Neologisms Database

Etymology : On Twitter, to mention someone and start a public conversation with them, you put the @ symbol before their username, which notifies them that you sent them a message. Most Twitter conversations start because of disagreements between strangers, which leads in the original person who made the tweet becoming annoyed that someone is arguing with them and attempting to disparage their views and evidence. The phrase “@ me” originally referred to the act of mentioning someone in a tweet so that users could argue with the original person’s statement, but this phrase has spread beyond the domain of social media into real life. This has led to the @ symbol being replaced by the word “at” in writing, though @ is still acceptable. Atting someone underwent pejoration, switching from meaning simply mentioning someone to arguing with them online, before broadening to the domain of casual disagreements outside of social media.