A complete numpty who is too stupid and inept to realise just how stupid and inept he really is. Also thinks he's a bit clever - usually a bit of a slap-head as well...
A potatohead (abreviation: KK, from german "Kartoffelkopf") is a human with a potato-shaped head. However, the potato shape of the head is not disadvantageous to his / her prettiness, but rather an unique feature of the respective human's beauty.
A potatohead is usually owned by a potatoheadowner (abreviation KKB from german "kartoffelkopfbesitzer"), who normally is a male person, as women usually rather match the criteria of a KK.