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.