In Japan, when there’s something strange — and I mean strange — in your neighborhood, it might be the work of a 妖怪 (yōkai).

The word 妖怪 features two kanji that can both mean “strange” when used in the words 怪しい (ayashii) and 妖しい (ayashii). The kanji compound was first recorded in the “続日本紀” (“Shoku Nihongi,” 797), a text that posited there were always 妖怪 present in the courts. Something rotten in the state of Denmark, indeed.

At the time, 妖怪 did not refer to beings but to strange phenomena. Whether they were supernatural or mundane is unknown, but this usage of the term persisted through the Kamakura Period (1185-1333).