Known as 8th Grader Syndrome in the west. It is the result of trying to stand out by believing that one has special powers or something similar. An equivalent insult in English is calling someone "edgy".
Only use for images explicitly showing chuunibyou personality traits and clothes, such as dramatic hand gestures and expressions, eyepatches, and scarves. Do not use simply because the character is a chuunibyou in the series.
It's not meant as an abbreviation of the light novel series Chuunibyou Demo Koi ga Shitai!.