The day after Thor'sday Thursday and the arrival of the Green Death, Baggybum and the other Vikings of the Hairy Hooligan Tribe and Meathead Tribe gathered to discuss ideas to rid Berk of the giant dragon. He was involved at yelling at the dragon, which failed, and later was seen discussing better methods of yelling. At that time, his own son Snotface Snotlout burgled the hairy underwear off of him as he stood talking. His knickers were to be employed in Hiccup's idea of feather-bombing the Green Death, as well as the Purple Death.
As his name entails, Baggybum is very large. He is described as "about the size of a killer whale who hadn't had much excercise recently." in How to Be a Pirate.
Baggybum's underwear used to feather-bomb the Green Death