origin:  Japanese; neologism, 中二病 (ちゅうにびょう)= chuugakkou ni-nen= "2nd year of Middle School" + byou= illness.
A term coined by Japanese celebrity Hikaru Ijuin, chuunibyou is similar to the American concept of "The Wonder Years", akin to "weeaboo" and "poser". Like "otaku" (obsessed anime fan), the word has negative connotations of an annoying person more focused on themselves and their fantasies than anything else.
One rough translation for chuunibyou would be: Middle School Syndrome
Specifically 8th grade, when people are just separating from childhood into adulthood, and therefore experimenting a great deal with their place in the world. It takes the form of a person desiring to be special and therefore imagining themselves as something fanciful (often inspired by favorite fictional characters), dangerous, and/or cool.
People going through a chuunibyou phase may truly imagine themselves as a gangster, princess, wizard, or suffering from a medical disorder. They may experiment with odd religions, various deviant behaviors, secret identities, or simply appropriate them to appear more unique. They may adopt specific habits or forms of dress to accentuate these images and seem more sophisticated, while ironically making themselves look more like a: dork, spaz, try-hard or wannabe.
However, it's also very normal, passing with age and maturity.
In my opinion, it may also be the stuff genius & dreams are made of...I doubt there is a rock star, writer, or peace advocate born that didn't spend time in chuunibyou fantasies. The important thing is learning to balance such delusions with faith in yourself as you really are, flaws and all, otherwise you'll hurt others and stunt personal growth. A popular anime (cartoon) exploring this theme with humor is: Chuunibyou Demo Koi ga Shitai!