WordWeb Online

1. Not likely to be true or to occur or to have occurred
   "legislation on the question is highly unlikely"
   - improbable
    
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"
    
3. Having a probability too low to inspire belief
   - improbable, unbelievable, unconvincing

Derived forms: unlikeliest, unlikelier

See also: farfetched, implausible, last, likelihood, likeliness, outside, remote, supposed, unlikeliness

Antonym: likely