Fucktard
The bastardisation of two words: "fuck" and "tard" - the latter being a shortened version of "retard", a derogatory word to decribe those with lesser mental ability.  In use, it is an insult that can usually express two emotions at once - complete fustration and disbelief.  Frustration at the stupidity of someone, and disbelief that yes, they are actually that stupid. 
"He is such a fucking fucktard!"