Why was Charles martel nickname''the hammer''?

In October, 732, Charles met Abd-er-Rahman outside of Tours and defeated and slew him in a battle (the Battle of Poitiers) which must ever remain one of the great events in the history of the world, as upon its issue depended whether Christian Civilization should continue or Islam prevail throughout Europe. It was this battle, it is said, that gave Charles his name, Martel (Tudites) "The Hammer", because of the merciless way in which he smote the enemy.