The goutweedy lump is a quest item found during the My Arm's Big Adventure quest. At the start of the quest, My Arm the troll asks the player to obtain it from the cauldron at Death Plateau so that he can grow fresh supplies of the herb at the Troll Stronghold.
According to My Arm, a man was cooked by the troll cooks on Death Plateau, Ash and Dung. While searching his trousers they found something that had similar properties to goutweed, but was not goutweed (gout tuber) so they threw it into the cooking pot along with their captive. When he finished cooking and they ate him they discovered that he was flavoured like Goutweed.
This rumour must have reached My Arm or Burntmeat as My Arm has knowledge of it being in the cooking pot.