Caillou is a Canadian animated children's series that aired from 1997 to 2010, following the everyday adventures of a bald four-year-old boy. His constant whining became a running joke among older viewers, and short clips of his cries began circulating as reaction sounds on YouTube and TikTok. The waa in particular works as shorthand for childish complaining, which is why meme editors reach for it whenever a video needs an instant, universally recognised tantrum.