The Collaborative International Dictionary of English v.0.48:

Unpalatable \Unpalatable\
   See palatable.

WordNet (r) 3.0 (2006):

unpalatable
    adj 1: not pleasant or acceptable to the taste or mind; "an
           unpalatable meal"; "unpalatable truths"; "unpalatable
           behavior" [ant: palatable, toothsome]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "unpalatable":
   afflictive, bitter, disagreeable, dislikable, displeasing,
   distasteful, flat, flavorless, foul-tasting, galling, grievous,
   ill-flavored, inedible, insipid, loathsome, matchless, nasty,
   nauseous, off, offensive, only, painful, peerless, rancid,
   repugnant, savorless, sickening, sour, tasteless, thankless, thin,
   turned, ugly, unalluring, unappealing, unappetizing, unattractive,
   undelectable, undelicious, undesirable, unengaging, unenjoyable,
   unequaled, uninviting, unique, unlikable, unmatched, unparagoned,
   unparalleled, unpleasant, unpleasing, unrivaled, unsavory,
   untasteful, untasty, unwelcome, washy, watery, weak