Not much is known about Uncle Hagelin, other than he lived to an old age and was considered wise. He authored such philosophy as:
|“||He who hath smelt it, hath dealt it.
And if it was more than just gas, he will hath felt it in his pants.
|— Ruffnut and Tuffnut quoting Uncle Hagelin|
In the episode "Dire Straits", the Dragon Riders visit the Straits of Baldur to discover why the Traders are no longer sailing through it and on to Berk. A fowl stench arises and the Twins blame Snotlout for the smell, citing their Uncle Hagelin. The smell, however, is the breath of a Submaripper.