From A Wiki of Ice and Fire
|Book(s)||A Clash of Kings|
Weasel is a girl who lived in the Riverlands. Her real name is unknown. She was found by Yoren on the march north from King's Landing. She was named "Weasel" by Lommy who claimed she looked like one.
A Clash of Kings
Weasel was a survivor from a holdfast that had been raided. She was taken in by Yoren on his march north. She survived the assault by Ser Amory Lorch and his men on the holdfast. When the survivors were discovered by men under the command of Ser Gregor Clegane, she ran off into the woods and was not found again.
While at Harrenhal, Arya Stark took on the girl Weasel's nickname as an alias for herself.