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In gambling, snake eyes is the outcome of rolling the dice in a game and getting only one pip on each die. The pair of pips resembles a pair of eyes, which is appended to the term 'snake' because of the long-standing association of this word with treachery and betrayal. The dictionary of etymology traces that use of the term back to 1929, although it may be traced all the way back to the ancient Roman dice games, where 'Dogs' represented two ones. They referred to this as "the dog throw". In modern parlance, it refers to such a roll in any game involving dice. Snake eyes also refers to looking one way and passing the ball the other way in the game of Taps.
Because this is the lowest possible roll, and will often be a loser in many dice games, such as Craps, the term has been employed in a more general usage as a reference to bad luck. In many board games such as Monopoly, house rules may decide that rolling Snake Eyes (or any Doubles) leads to a bonus since they are rarely rolled.
Rolling two 6-sided dice gives you a 1 in 36 chance of getting a snake eyes each time you roll the dice.
Rolling two 6-sided dice 25 times gives a probability of .505532 that at least once, snake eyes will appear.