While a lot of casino terms refer to games, actions, or bets, arm refers to the players. The term is used in a game of craps and it is reserved for the players who supposedly have enough skill at throwing the dice that they can alter or fix the outcome of the game.
Whether this is actually true or it’s just superstition, you shouldn’t take offence if someone calls you an arm, as they are only complimenting your skill at throwing dice and thanking you for their lucky streak, if they are betting on you, of course.
However, one thing to keep in mind is that, in order to ensure that the outcome of every dice roll is random and that players aren’t cheating, casino craps dealers require that the dice are thrown against the furthest table wall.