Longman Dictionary of Contemporary English - sour

~1 adj 1 having a sharp acid taste that stings your tongue, like the taste of a lemon  (sour, tangy apples | Sprinkle a little sugar over the strawberries if they are sour.)  (- compare bitter1 (4), sweet1 (1)) 2 milk or other food that is sour is not fresh and has an unpleasant taste because it has fermented (ferment1)  (turn/go sour (=become sour))  (In warm weather, milk can go sour in just a few hours.) 3 unfriendly or looking bad-tempered  (Rob gave me a sour look. | sour-faced)  (a sour-faced old man) 4 sour grapes the attitude of someone who pretends to dislike something that they really want, because they cannot have it 5 turn/go sour informal if a relationship or plan turns or goes sour, it becomes less enjoyable, pleasant, or satisfactory  (As time went by their marriage turned sour.) - sourly adv - sourness n ~2 v 1 if a relationship or someone's attitude sours, of if something sours it, it becomes unfriendly or unfavourable  (An unhappy childhood has soured her view of life.) 2 if milk sours or something sours it, it begins to have an unpleasant sharp taste