Pinhead

[Vania]

One who lacks the intelligence of the normal [sector] of the human population; even so cannot handle the most [mundane] of tasks due to the lack of [common sense] and intelligence.

[Jason Yan]

The round head of a pin.

[Magaret]

someone who is [clueless] and has [no idea] of what so [ever].

[Donald Trump]

Lead cynobite in the [horror flick] [Hellraiser] by [Clive Barker].

[Little_Girl]

the very small round end of a pin:

[Monty]

The flattened head of a pin.