sour grapes.
This is one of the most commonly misused idiomatic metaphors. It is not a mere synonym of "envy" or "jealousy." Rather, as in Aesop's fable about the fox who wanted the grapes he could not reach, "sour grapes" denotes the human tendency to disparage as undesirable what one really wants but can't get (or hasn't gotten). For example, a high-school boy who asks a girl for a date and is turned down might then insult her in all sorts of puerile ways. That's a case of sour grapes.
But the traditional and correct use of the phrase seems to be on the wane. Some uses are downright incoherent — e.g.: "Great Britain's reaction [in the Falklands War] was more a case of sour grapes and wounded pride than any genuine desire to right a terrible wrong." Letter of Philip Naff, "Falklands Furor," Time, 10 May 1982, at 5. (The British reaction couldn't have been "sour grapes" because [1] Great Britain did not disparage the Falklands as undesirable — it wanted to keep them as a territory; and [2] Britain was successful in the effort.)
The more typical misuse looks like this: "Is someone trying to jinx Good Will Hunting's chances for a screenwriting Oscar? Perhaps a competitor's sour grapes [read 'envy'] over the film's success?" Nick Madigan, "Bad Vibes Haunt 'Good Will' Nom," Daily Variety, 16 Mar. 1998, at 36.
Language-Change Index — "sour grapes" in the sense "envy": Stage 3.
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Quotation of the Day: "We know for a fact that something like 'written speech' is one characteristic that marks off mid-twentieth-century writing from the prose of the past." A.P. Rossiter, Our Living Language 101 (1953).