At the craps table, “arm” is a nickname for a shooter who supposedly throws the dice with such control that they can steer the result. Players who are winning on a shooter’s rolls may call them a good arm as a compliment.
Whether dice control works at all is disputed, and casinos make it hard in practice. House rules usually require both dice to hit the far wall of the table, which is lined with a rubber pattern of small pyramids that deflects the dice unpredictably.
The term sits alongside other streak slang such as hot and ideas like the gambler’s fallacy: each roll is independent, whoever throws it. Online craps removes the question altogether, because the results come from a random number generator rather than a human hand.