Definitions of sour:

• noun:   the taste experience when vinegar or lemon juice is taken into the mouth
• noun:   a cocktail made of a liquor (especially whiskey or gin) mixed with lemon or lime juice and sugar
• noun:   the property of being acidic
• verb:   make sour or more sour
• verb:   go sour or spoil

  Example: "The milk has soured"

• adjective:   having a sharp biting taste
• adjective:   one of the four basic taste sensations; like the taste of vinegar or lemons
• adjective:   smelling of fermentation or staleness
• adjective:   in an unpalatable state

  Example: "Sour milk"

• adjective:   inaccurate in pitch

  Example: "A false (or sour) note"

• adjective:   showing a brooding ill humor

  Example: "A sour temper"

• name:  A surname (very rare: popularity rank in the U.S.: #49287)