Why is it a negative term, you ask? Going back to the Charlie Brown days.... Charlie Brown was pretty much considered a born loser in life. So Sally said as she duped him time and time again w/ the football kick off. Talk about a guy w/ a low level of self confidence!
A google brought this up also:
A vintage insult commonly used by older generations. Commonly directed towards anyone that demonstrates low intelligence or more specifically, very low common sence (sense). Grandfathers and Great-Grandfathers of 60's to 90's born generations would utilize this word.
You're such a blockhead, Charlie Brown!