From A Wiki of Ice and Fire
|Book||The Sworn Sword (appears)|
Will, called Treb, was a farmer in service to House Osgrey.
When smallfolk were called up to defend House Osgrey lands after a dispute over the Chequy Water with House Webber escalated, two men named Will showed up. When one of the Wills said he could throw rocks far, everyone started calling him "Treb" once Ser Bennis of the Brown Shield referred to him as a trebuchet.
Treb was sent home with the others after Ser Duncan the Tall realized the weakness of their position.