Coming from the psychological horror Anime game Doki Doki Literature club. This term is generally used when a typically innocent show, game, anime or cartoon takes an unexpectedly dark and disturbing turn.

Japanese word that refers to the sound of a heart beating

The pitter-patter of the heart when around their senpai

The depression or sadness felt after playing the game Doki Doki Literature Club.

Also known as Yume Koujou: Doki Doki Panic, this game was originally actually supposed to be a Mario game in Japan, but it got changed to a game about an Arabian family going inside a book. This later became Super Mario Bros. 2, for the U.S. The reason this become Mario 2 was because Nintendo thought that the game made for Japan was way too hard (you can't really blame them), even though it did introduce a few mechanics that are used in some Mario games today. The U.S. version of the game actually did see a release in Japan, known as Super Mario USA.

Also, the U.S. version did use some characters which became standard in some Mario games (just look at Birdo, and the ability to play as Toad).
According to tcrf, Doki Doki Panic was originally created as an advertisement for a "Dream Factory" in Japan (That is what Yume Koujou translates into).

doki doki morning is a debut song from a japan kawaii metal group BABYMETAL

A fan-made song for Doki Doki Literature Club, some of the creators are Oreo and Chi-Chi.

A game that's really messed up. Don't be fooled by the cute exterior

Hell.