that is so far over the line, It's purely retarded
and idiotic, and is funny.
Usually this kind of humor is aided by ridged and awkward body movements, like the classic "curl the wrist" retard stereotype, along with some words and it's funny.
Only a handful of people have this kind of humor, but when you do, everything is extremely funny.
Exaples of Retarded Humor.
Me: YO DUDE WANNA EAT MY PENCIL
Friend: IS IT SALTY
Me: POTATO FUCKS