adjective

1. having the sharp, acid taste of lemon juice, vinegar, green fruit, etc.
2. made acid or rank by or as by fermentation sour milk
3. 1. cross, bad-tempered, peevish, morose, etc. a sour mood
   2. ill-disposed and bitter sour toward former associates
4. below what is usual or normal; poor; bad his game has gone sour
5. distasteful or unpleasant
6. gratingly wrong or off pitch a sour note
7. excessively acid: said of soil
8. tainted with sulfur compounds: said of gasoline, etc.

Etymology: ME soure < OE sur, akin to Ger sauer, ON sūrr < IE *suro-, sour, salty > Latvian sũrs, salty, bitter

noun

1. that which is sour; something sour
2. ☆ a cocktail made with lemon or lime juice, sugar, and, usually, soda water a whiskey sour

transitive verb, intransitive verb