plonker
the term plonker started out as a reference to someone who was forever drunk on cheap wine (cheap wine is nicknamed plonk) this person was usualy a homeless person, or poor person.

today the term plonker is a very light hearted insult. Its not even seen as an insult, its like calling someone a wally. Its in no way ment maliciously. you call someone a plonker when they say or do something stupid.
bob - why didnt my food cook ?
john - you never turned on the oven, you plonker.