From 1d4chan

Mao, sometimes called "that game" or "the game of our great leader," is a card game and metagame. If you were to watch a game, you would see some very quiet, intense people holding hands of cards -- the players with a lot of cards would look pretty frustrated -- with a draw deck and discard deck between them. Players would each play one card a a time, or pick up a card from the draw deck, sometimes shouting out a non-sequitir, or sometimes another player will take a card from the draw pile and give it to another player. The players won't answer questions, won't talk to you, and won't talk to each other. Sometimes a stranger will walk up, grab some cards off the draw pile, and then won't talk to you either.

There are many variations of Mao, as the game mutates when it moves from university to college to campsite to whereever. Here's the rules to Mao:

  1. A player's turn consists of picking a card up from the table or putting a card down from their hand.
  2. You are not allowed to tell anyone the rules of Mao beyond the first two rules, even when you're not playing the game.
.... uh, well, shit. Nevermind.

When you play the game, you'll have to figure out the local rules as you go along. The people you play with will help you learn the rules (for certain values of "help")... and the rules might change during the game. Sorry, I can't tell you how; see rule #2 above.

TLDR; its fucking Calvinball as a cardgame where the goal is to find out the rules of the game and laugh as others try to blunder their way into finding out the rules. Although there are a "standard" set of rules, each social group's version of Mao should be different.

Card Games
Card Games:
Call of Cthulhu - Cardfight!! Vanguard - Fire Emblem Cipher
Force of Will - Jyhad - Magi-Nation Duel - Magic: The Gathering
Netrunner - Pokémon - Star Wars: Destiny CCG (Dead) - Yu-Gi-Oh
Card Games:
1000 Blank White Cards - 7th Sea - Apples to Apples - Bang!
Cards Against Humanity - Coup - Decktet - Dominion - Dvorak
F.A.T.A.L. - Mafia - Mag Blast - Mao - Munchkin
Race for the Galaxy - Sentinels of the Multiverse - Tanto Cuore
Card Games:
Bridge - Cribbage - Mahjong - Solitaire/Patience - Poker - Rummy - Tarot