From A Wiki of Ice and Fire
Lion's Tooth is the name Joffrey Baratheon gives his sword.
A Game of Thrones
While out riding with Sansa Stark, Joffrey comes across Arya Stark and the common boy, Mycah, playing at swords. He threatens Mycah, which leads Arya to attack him. Joffrey is bitten by Nymeria and Arya throws Lion's Tooth into the Trident. Arya would later misremember the sword's name as "Lion's Paw".,