Arm

In dice games, such as craps, an arm is a term used for someone who has the skill of throwing the dice in such a way as to alter the odds. This means that, in theory, such a person would be capable of throwing the dice in a way that would consistently achieve a rare (and highly-paing) outcome. 

Whether or not such players exist in real life is debatable, with some considering all mentions of them as being nothing more than urban legends.

Arm:

• A dice player who is capable of consistently achieving rare outcomes
• Considered an urban legend by some