The phrase that's cap comes from internet slang for calling something a lie, and this clip pairs it with Soulja Boy's delivery, which is why it is catalogued under his name. Short reaction sounds like this spread through meme pages and short-form video edits, where a single line can stand in for a whole response. It remains popular because the wording is instantly understood by younger audiences and the delivery sounds confrontational without needing context, making it easy to reuse across gaming, lifestyle and comedy clips.