The story
From plonk (“(noun) sound of something solid landing; (verb) to set or toss (something) down carelessly”) + -y (suffix forming adjectives with the sense ‘having the quality of’)..
Etymology from Wiktionary, CC BY-SA 4.0.
plonky
Today it means: Chiefly of pianos: having an unappealing, thudding sound. Full definition →