The humorous expression Château Plonk, also Chateau Plonk, and its variants, are used of wine, especially of cheap wine of inferior quality.

This expression occurs, for example, in A little Hungarian Muscat love, by Michael Vaughan, published in the National Post (Toronto, Ontario, Canada) of Saturday 2^nd June 2001:

Unfortunately, many of us naturally equate quality and price, which is expected since pedigree is what supposedly drives price. This tends to make snobs of us all; we prefer Château Mouton Rothschild to Château Plonk, even though the latter may be a bargain. We fawn on Australian whites, disdaining those from, say, Hungary.

The expression Château Plonk, also Chateau Plonk, is based on the contrast in signification between the two words that compose it:
1) The noun château, as used in names of expensive wines of superior quality made at vineyard estates, especially in the Bordeaux region of France—as in Château Margaux, Château d’Yquem, Château Lafitte and Château Latour.
2) The noun plonk, denoting cheap wine of inferior quality.

—Cf. also the humorous expressions chateau tap-water and Château-la-Pompe, which liken tap-water to a grand cru.

The following illustrates the contrast between the noun château and the noun plonk—it is from an account of the trial in Bordeaux, France, of eighteen wine traders accused of doctoring or mislabelling nearly three million litres of wine, published in The Journal (Newcastle-upon-Tyne, Northumberland, England) of Wednesday 30^th October 1974:

They can’t tell plonk from chateau

INTERNATIONAL wine distributors admitted yesterday at the “Winegate” trial in Bordeaux they sometimes could not tell the difference between cheap and quality wines.

The earliest occurrences that I have found of the expression Château Plonk, also Chateau Plonk, and variants, are as follows, in chronological order:

1-: From Liquid Assets, by Bob Phillips, published in the Coventry Evening Telegraph (Coventry, Warwickshire, England) of Tuesday 18^th December 1973:

THE SEASON of goodwill is almost upon us. Liberal quantities of vin ordinaire, and some not so ordinaire, will be sloshed down. Over lunch-tables the length and breadth of the country the brandy and port will be brought out for their annual airing.
It’s worth a moment’s reflection before you drain the dregs of your Chateau Plonk. The wine business these days has become as much a province for commercial investors as for those who simply enjoy a taste at the grape—though surprisingly, with a dozen bottles of claret on a cellar shelf worth upwards of £300, it’s a trend wine merchants themselves deplore.

2-: From the title given to a letter, from one Albion F. Watkinson, published in The Observer (London, England) of Sunday 20^th November 1977:

Chateau Plonk

The widely observed convention that restaurateurs offer the sealed bottle of wine for inspection and open it at the table is a necessary protection for the consumer, who in paying quite a high price for a dinner wine, perhaps £4 for a bottle, does not expect to be served cheap and indigestible ‘plonk’ disguised as the wine of his choice by having been poured into an empty bottle of that vintage!

3-: From Blank men, blank menus, an article by Joan Bates on how women are treated in hotels and restaurants, published in The Birmingham Post (Birmingham, Warwickshire, England) of Saturday 22^nd July 1978:

Women are always on a diet. They are rumoured to be mean with tips, to buy one glass of house wine instead of two bottles of Chateau la Plonk de Plonk 1972, followed by crusted port and cigars.

4-: From Get set for election as soon as February, by Richard Gwyn, published in the Ottawa Journal (Ottawa, Ontario, Canada) of Saturday 16^th December 1978:

Now that Prime Minister Trudeau has put an end to the rumours that he is about to resign, it is time to start up another one.
The next election campaign may begin amid the snowdrifts of February or March, rather than, as everyone now assumes, amid the mud and slush of spring.
If, after the holly and the turkey and the Chateau de Plonk, you have some spare cash lying around, you might lay out a small bet on an early election.

5-: From A masterly choice of wine…, by Janet Buckton, published in the Coventry Evening Telegraph (Coventry, Warwickshire, England) of Tuesday 17^th July 1979:

PICK UP a bottle of “chateau plonk” when you buy your pound of cheese this weekend.
That’s the message from British Home Stores, the latest store to move into the wine market.
But far from “plonk” they’ve concentrated on good quality wines, in the middle to upper-middle range.

6-: From Guardian Diary, published in The Guardian (London and Manchester, England) of Thursday 19^th June 1980:

Mrs Thatcher’s fear that her settlement of the budget issue and associated foreigner-bashing […] has left all the electoral running on the EEC in Labour’s hands. Europe is recognised as an undoubted vote-loser with its current image as a charitable society for the French, built on the slopes of a butter mountain beside a lake of Chateau Plonk.