Beefwitted Barnacle

Someone with the intellectual capacity of a cerebrually defective cow and the mannerisms of a parasite.
In other words, an idiot who leeches off of other people's resources without contributing anything in return.
In OTHER other words, the bad funcle and your 26 year old cousin who lives in their mother's basement.

Also used as a general insult when someone does something stupid.

Jimmothy: *spills the tea kettle on Pathew* Whoops
Pathew: OW wtf Jim! You beefwitted barnacle!