1. [of something] very sour. (*Also: as ~.) This milk is as sour as vinegar. The juice they gave us is sour as vinegar.
2. [of someone] ill-natured and disagreeable. (Fig. on {2}. *Also: as ~.) The old man greeted us ill-naturedly, his face as sour as vinegar. Jill: Is Mary in a bad mood today? Jane: Yes, sour as vinegar.