1. A slang term used in Britain for penis.
2. A man whose girlfriend sleeps with his male friends.
3. A fool or inept person who is easily taken advantage of, popularized in the British TV series “Only Fools and Horses” where the main character played by David Jason often used the phrase “Rodney, you plonker!” when talking to his naive younger brother.
“Don’t be such a plonker!”
“You got taken for a ride, you plonker!”