Words at Play : Top 10 Favorite British Words, Vol. 1
Definition:
cheap or inferior wine
Example:
"Fine diners are drinking premier cru wines at plonk prices as a bring-your-own booze revolution gathers pace in Britain's best restaurants." Robert Booth, The Guardian, June 20, 2010
About the word:
Plonk is a shortening of the earlier plink-plonk, and it may be a modification of vin blanc, which means "white wine" in French.