Grandpa Stinky is a side character in Sam & Max Season Two.
Stinky is the original proprietor of Stinky's Diner, which he used to engage in gastronomical experiments to better the human race (mostly by creating food so bad only the toughest can stomach eating it). One such creation was The Cake of the Damned, better known as Girl Stinky, who believes him to be her grandfather. Girl Stinky took over the diner after the old man died in a mountaineering accident, which ironically could have been prevented by the superior adhesive Girl Stinky would have invented if Sam & Max hadn't traveled back in time to steal the family recipe in Chariots of the Dogs. Still, Sam & Max bring him back from the dead in What's new, Beelzebub?, after which the Stinkys jointly run the place.
Sam and Max have fond memories of the old man and much prefer him to the new Stinky, but their enthusiasm doesn't last long when they actually meet him again in Chariots of the Dogs.
The Stinkies re-appear throughout Season Three. They still run the place together, but argue a lot. It is frequently hinted that Stinky keeps trying to kill Grandpa Stinky throughout the season, culminating in The City That Dares Not Sleep when she has his brain swapped for the one of one of Skun-Ka'pe's minions in order to have Flint Paper kill him without knowing his identity. The plan was foiled by Sam, who pointed out to Flint which one Grandpa Stinky was. In the ending, Grandpa Stinky still has a gorilla body, wearing his trademark captain's hat.Stinky's arms bear a tattoo featuring Norse Futhark runes spelling out what transliterates into Satang. Thus far the possible significance of this has only been speculated on.