WordNet (r) 3.0 (2006):

long shot
    n 1: a venture that involves great risk but promises great
         rewards
    2: a contestant that is unlikely to win

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "long shot":
   accident, adventure, bare possibility, boom shot, casualty,
   chance hit, close-up, contingency, contingent, dark horse,
   deuce shot, equivalent odds, even break, even chance,
   fighting chance, fluke, follow-focus shot, fortuity,
   freak accident, full shot, gambling chance, good chance,
   group shot, hap, happening, happenstance, hardly a chance, hazard,
   hundred-to-one shot, improbability, kinescope, little chance,
   little opportunity, long odds, lucky shot, matte shot, medium shot,
   motion picture, no chance, odds, off chance, outside chance,
   pan shot, panoramic shot, poor bet, poor lookout, poor possibility,
   poor prospect, price, process shot, rap shot, remote possibility,
   retake, short odds, shot, slim chance, small chance, square odds,
   take, travel shot, trucking shot, unlikelihood, zoom shot