The Collaborative International Dictionary of English v.0.48:

Unlikely \Un*like"ly\, adv.
   In an unlikely manner.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unlikely \Un*like"ly\, a.
   1. Not likely; improbable; not to be reasonably expected; as,
      an unlikely event; the thing you mention is very unlikely.
      [1913 Webster]

   2. Not holding out a prospect of success; likely to fail;
      unpromising; as, unlikely means. --Hooker.
      [1913 Webster]

   3. Not such as to inspire liking; unattractive; disagreeable.
      [Obs.] "The unlikely eld of me." --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unlikely
    adj 1: not likely to be true or to occur or to have occurred;
           "legislation on the question is highly unlikely"; "an
           improbable event" [syn: improbable, unlikely] [ant:
           likely, probable]
    2: has little chance of being the case or coming about; "an
       unlikely story"; "an unlikely candidate for reelection"; "a
       butcher is unlikely to preach vegetarianism" [ant: likely]
    3: having a probability too low to inspire belief [syn:
       improbable, unbelievable, unconvincing, unlikely]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "unlikely":
   distasteful, doubtable, doubtful, dubious, dubitable,
   hardly possible, implausible, improbable, inappropriate,
   inauspicious, inconceivable, incredible, more than doubtful,
   objectionable, questionable, remote, unacceptable, unattractive,
   unbecoming, uncongenial, unfit, unfitting, unimaginable,
   unpromising, unpropitious, unseemly, unsuitable, unthinkable