An arm was a body part.
According to Faction Paradox legend, Grandfather Paradox had only one arm. (PROSE: The Gallifrey Chronicles) A future version of the Eighth Doctor whose biodata had been corrupted by the Faction modelled himself as Grandfather Paradox, including removing one of his arms. This also may have been because he knew it was the arm which the Doctor would use to destroy Gallifrey. (PROSE: The Ancestor Cell)