Almost any penis, but not a very small one.
A foolish person.
A big wet kiss.
Almost anything that is remarkably big.
Almost certainly most derive from plonker as size, but this in turn possibly from "to plonk", to place with little or no accuracy possibly because it was big and heavy. See also plonka
"Put your plonker away, Eric."
"Oh Bob, you're such a plonker."
"When I said she looked nice, she gave me a plonker straight on the lips."
"It was the biggest turkey we'd ever had. A real plonker."
"She plonked the meal down in front of me. 'Get stuck in,' she cried."