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.
A song written by The Ramones (specifically by Dee Dee Ramone) in 1977 on their "Leave Home" album. The lyric goes: "I don't wanna be a pinhead no more. I just met a nurse that I could go for...D-U-M-B Everyone's accusing me!" The song was inspired by the movie "Freaks" and the comic "Zippy the Pinhead." The word "pinhead," in this context, refers to an outcast/freak/childish character with a cone-shaped head.
"Jo is wearing an outfit that looks different from what everyone else in the school is wearing. What a pinhead."
"Jo's head is pretty pointy at the top. I wonder what it must be like to be a pinhead."
O'Reilly (eyes expanding): - OKAY, CUT THE MIC! CUT IT!
O'Reilly (calming down): Those pinheads really are hatfeul. But, at Fox News since we are 'fair and balanced' we have to give them their say. It's hard being fair. (smiles)