sour / ˈsou(ə)r/ • adj. having an acid taste like lemon or vinegar: she sampled the wine and found it was sour. ∎  (of food, esp. milk) spoiled because of fermentation. ∎  having a rancid smell: her breath was always sour. ∎ fig. feeling or expressing resentment, disappointment, or anger: she was quite a different woman from the sour, bored creature I had known. ∎  (of soil) deficient in lime and usually dank. ∎  (of petroleum or natural gas) containing a relatively high sulfur content.• n. a drink made by mixing an alcoholic beverage with lemon juice or lime juice: a rum sour.• v. make or become sour: [tr.] water soured with tamarind | [as adj.] (soured) soured cream | [intr.] a bowl of milk was souring in the sun. ∎  make or become unpleasant, acrimonious, or difficult: [tr.] a dispute soured relations between the two countries for over a year | [intr.] many friendships have soured over borrowed money. PHRASES: go (or turn) sour become less pleasant or attractive; turn out badly: the case concerns a property deal that turned sour.sour grapes an attitude in which someone disparages or affects to despise something because they cannot have it themselves: government officials dismissed many of the complaints as sour grapes.DERIVATIVES: sour·ish adj.sour·ly adv.sour·ness n.

sour sour grapes an expression or attitude of deliberate disparagement of a desired but unattainable object, alluding to Aesop's fable of ‘The Fox and the Grapes’, in which a fox, unable to reach a tempting bunch of grapes, comforted himself with the reflection that the fruit was probably sour and was therefore no loss.

See also the grapes are sour.

sour of tart or acid taste OE.; bitter, extremely distasteful XII; morose, peevish XIII. OE. sūr = OS., OHG. sūr (Du. zuur, G. sauer), ON. súrr :- Gmc. *sūraz.
Hence sour vb. XIV. sourness OE. sūrnes.