"The Strange Case of Dr. Jekyll and Mr. Hyde" is a novella by Scottish writer Robert Louis Stevenson, published in 1886. The names of Dr. Jekyll and Mr. Hyde, the two alter egos of the main character, have become shorthand for the exhibition of wildly contradictory behavior, especially between private and public selves. The notion of the “double” was widely popular in the 19th century, especially in German literary discussions of the doppelgänger.

Source: Britannica