To set something down heavily or carelessly with a dull sound; or cheap wine of poor quality.
Possibly onomatopoetic from the sound something heavy makes when dropped. As a term for cheap wine, it originated in Australian English around the 1960s.
Australians coined 'plonk' for cheap wine and plonking it on tables—the word's heaviness perfectly captures both the action and the unrefined nature of bulk-produced wine.
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