Indeed, the correct spelling is "doofus". Any Doofus knows that! Here's the definition from the urban dictionary:
Someone who hasn't got a clue!
They live in blissful ignorance of the world, fashion, personal hygiene and social skills.
Invariably laughs like a baboon being repeatedly punched.
"That bloke is a total dufus"
"He laughs like a fat, useless dufus! Lets promote him!"