Roman soldiers carried spears but the day after Jesus was crucified, a soldier, identified in the apocryphal Gospel of Nicodemus as a Centurion and called Loginus, attended the scene with another weapon; a club. His task was to break the legs of any victim hanging on a cross who was still alive.
The purpose of breaking legs was to make it impossible for the condemned to push up their torso against the suppedaneum in order to breathe; breaking legs would suffocate the victim. It was critical that none of the victims were still alive to defile the Sabbath by dying at the wrong time. But when Loginus approached, Jesus appeared to have already expired.
To make sure of this, Loginus pierced the side of Jesus with a spear1. Unbeknownst to Loginus, he was fulfilling a prophecy2, hence the spear's inclusion in the Arms of Christ.
From this wound flowed "blood and water". Although Jesus was presumed already dead at this time, the popular inference of the blood and water is that Jesus died of a broken heart. The blood is seen by Christians to represent redemption and the water represents baptism.
The spear is known as the Holy Spear (or Lance), - of Destiny, or of Longinus, or of Christ.
The spear is a weapon, used for hunting food or against an enemy. It is a Christian's duty to hunt truth and to fight evil. For this reason, the spear is a shown upright, ready for use. (See in contrast the Warrior's Cross.)