plonk
n. UK //plˈɒŋk// plonk
Onomatopoeic. Compare plunk.
From WWI military slang, derived by alteration of French vin blanc (“white wine”) by the law of Hobson-Jobson. Recorded earliest in the playful rhyming slang form plinketty-plonk. Possibly influenced by the sound of wine being poured into a glass.
Probably a shortening of plonker.