The Collaborative International Dictionary of English v.0.48:

Unsweet \Unsweet\
   See sweet.

WordNet (r) 3.0 (2006):

unsweet
    adj 1: (of champagne) moderately dry [syn: sec, unsweet]
    2: distasteful; "he found life unsweet"

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "unsweet":
   acerb, acerbate, acerbic, acescent, crab, crabbed, dry, green,
   pickled, pungent, sec, sour, sour as vinegar, soured, sourish,
   tart, tartish, unripe, unsweetened, vinegarish, vinegary