The Collaborative International Dictionary of English v.0.48:

Sourish \Sour"ish\, a.
   Somewhat sour; moderately acid; as, sourish fruit; a sourish
   taste.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sourish
    adj 1: tasting sour like a lemon [syn: lemony, lemonlike,
           sourish, tangy, tart]