ATM is not a formal slang, but a casual acronym. It is common on the internet, but it is mostly applied in casual communication. Contrary to its other sense, Automated Teller Machine, in this case, serves as a short hand in day to day life. It is not typical of professional or scholarly writing, but it is quite all right in text messages, group messages, and comments on social media. ATM is a convenient digital form of expression as people can save time and exchange quick updates without typing entire sentences. Know more about atm meaning in text