Final answer:
The crossbow was outlawed by the Lateran Council of 1139.
Step-by-step explanation:
The weapon that was outlawed by the Lateran Council of 1139 was the crossbow. The crossbow was a powerful weapon that used a mechanism to shoot projectiles, making it effective in warfare. However, it was seen as a threat to knights and the chivalric code, as it could be operated by foot soldiers and peasants, diminishing the advantage of the heavily armored cavalry.