Definition

The phrase became popular in online communities and hip-hop culture in the 2010s, then carried over to social media, where people use it as a quick way to show they're serious about what they do or believe in. You will see "ATL" pop up in gaming chats, rap lyrics, or casual group texts, usually to call out or hype up someone who's proving themselves. For example, if a friend stays out late partying and still shows up for work the next day, someone might say, "Okay, you're really ATL."

Origin

slang.net