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.
plonker definition by Urban Dictionary
