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.
Billy: "Hey dad i microwaved the cat to dry his fur after the bath and he just exploded wtf"
Dad: "You're a fucking brainlet Billy"