brainlet

immature,underdeveloped cerebral cortex,unfit for real world survival

somebody who has a very small brain and can't make simple decisions

Someone associated with having a low iq and a below average common sense.

Individuals that search their names on Urban Dictionary and share the definition with others in order to feel good about them seleves. These people generally have IQs equivalent to that of small fish and think that Lelepons is funny.

Someone with severe mental retardations.
Just like a manlet is a half-man, a brainlet is someone with half a brain or endowed with a very small amount of grey matter.

Someone so mentally retarded it hurts

a complete moron, someone so stupid it's unbelievable they've not been killed off by their own stupidity