The Collaborative International Dictionary of English v.0.48:

Guesswork \Guess"work`\, n.
   Work performed, or results obtained, by guess; conjecture.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guesswork
    n 1: an estimate based on little or no information [syn:
         guess, guesswork, guessing, shot, dead reckoning]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "guesswork":
   actuarial prediction, apocalypse, assumption, axiom, conjecture,
   foreboding, forecast, forecasting, foreshowing, foresight,
   foretelling, hypothesis, improbability, inference, omen, postulate,
   postulation, postulatum, prediction, prefiguration, prefigurement,
   prefiguring, premise, presage, presaging, presentiment, preshowing,
   presignifying, presumption, presupposal, presupposition,
   probability, prognosis, prognostication, promise, prophecy,
   prophesying, proposition, prospectus, set of postulates, soothsay,
   speculation, statistical prediction, supposal, supposing,
   supposition, surmise, thesis, vaticination, working hypothesis