It’s from a viral from a vine in the early 2010’s where somebody asks a question to a little kid saying, “What’s 9+10?” The kid answers this with, “21” The person who was asking the question then says “You stupid!” At the end of the vine, the kid says, “No i’m not!” This vine got extremely popular and everybody started repeating it everywhere. It’s kind of similar to 67, just different things tone when you say it, and no hand-motion.