Eighth-grader Syndrome (or Chūnibyō)

the Eighth-grader syndrome originated in Japan where it is known as Chūnibyō ('Middle [School] 2[nd Year] Syndrome'). It is a colloquial term typically used to describe adolescents with delusions of grandeur. These teenagers are thought to desperately want to stand out and convince themselves that they have hidden knowledge or even secret powers.

In the United States the syndrome is usually in the context of localizations of anime which feature the concept as a significant plot element.

Chuunis, the patient who suffers from Chūnibyō, act like overly melodramatic Know-Nothing Know-It-Alls who think of themselves as more awesome than everyone else because of experiences, abilities, and even special powers that they clearly don't have.

The origin of the term is quite recent, and was first used by Japanese comedian Hikaru Ijūin in November of 1999, who described the effect as if it were an illness he had contracted[1]. He originally used it much more broadly, to refer to any kind of childish thinking or behaviour exhibited by children of that age.

In 2008, Hyōya Saegami wrote a book called Chūnibyō Toriatsukai Setsumei Sho ('Chūnibyō User Manual'), in which he identifies three types of chūnibyō: [1] DQN, who act like delinquents; [2] subculture, who go against the mainstream trends; and [3] evil eye, the most prevalent, who aspire to have special powers.

In 2009, Ijūin made a statement disavowing the idea, as it had changed from a light-hearted remark to a condition that was studied seriously in psychology. Overnight, it began to explode as a cultural idea and then became one.

In the Western world, this syndrome might be akin to juvenile or early-onset Napoleon Syndrome.

[1] Nagareboshi: Chuunibyou: Funny or Something Darker? in Honey's Anime – 2017. See here.