Zettai Zetsumei Dangerous Jiisan (2004)

Synopsis

Grandpa Danger is a world where the great protector is Grandpa Danger! He's an expert at falling off of cliffs, dealing with a principal who suffers from a severe inferiority complex, and escaping the hidden dangers of sleep! He's a nonsensical, utterly disgusting, relentless prankster. Grandpa Danger's excessive phlegm and farts frustrate Grandson to hilarious extremes, and make you laugh until your stomach splits!

(Source: AniDB)