The game of Mao is a member of the Eights group of card games. It has been popular with college students for well over 25 years. Part of its attraction is the mystery that shrouds it.

It is likely that Mao is a direct descendant of the German game Mau-Mau, which works on similar principles. There is also some speculation that Arthur Machen's short story The White People has some links to it:

I must not write down the real names of the days and months which I found out a year ago, nor the way to make the Aklo letters, or the Chian language, or the great beautiful Circles, nor the Mao Games, nor the chief songs.
The story was originally written in 1899 - which is likely to be several decades before the card game was invented. It is possible that whoever invented the card game had read the story and was influenced by it.