The word is 'doofus' and it is thought that it may have originated with a blended Scottish word for fool (doof) and the word goof for a mistake.
Dufus is also recognized in most slang dictionaries. So you can have it both ways if you'd like.
Personally I prefer the more descriptive 'dimwit' or 'nincompoop'
"Always smile at your enemies. It will keep them on their toes"