Game variation on liar's dice (see liar's dice).
The variation is a 2,1 is an "instant drink" for the next player.
If the next player believes you, then they drink. If they do not and you are lying, then YOU drink twice. If you are telling the truth, then HE/SHE drinks twice as much.
The twenty one is bets told to the next player by humming a familiar Mexican tune like "La Cucaracha" or "Mexican Hat Dance".
Also can be played with a shot of booze as the penalty/reward for having Mexican.
Mexican Dice is played like this (read liar's dice first):
Player 1: (shake, sees a 4,5) Poorly singing "La cucaracha, la cucaracha..." "Mexican!"
Either
Player 2: Says "OK" (drinks the shot of booze)
-or-
Player 2: Says "Bullshit" (lift the cup to see the 4,5). So, Player 1 drinks the shot of hard booze, or whatever.
The term "I'm so dice" is a term commonly used by gamers to mean "I'm so cool" or "I'm so raw. It was originally created in the early years of D & D to be something one would say after rolling a critical hit, or the highest number on the dice, as if one were saying, I'm the best!" As gaming evolved and video games became more popular, it became popular for gamers all over to say I'm so dice whenever anything cool or amazing happened in a game that allows one to show off their superiority in that game, such as getting an amazingly high kill streak in Call of Duty, or being able to raid all dungeons in WOW. I'm so dice is really just a cool, popular term. It's so dice!
Gamer 1 (after seeing his friend get a 75 kill streak in COD): Dude! You just got a 75 kill streak!