The Collaborative International Dictionary of English v.0.48:

Speculate \Spec"u*late\, v. t.
   To consider attentively; as, to speculate the nature of a
   thing. [R.] --Sir W. Hamilton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Speculate \Spec"u*late\, v. i. [imp. & p. p. Speculated; p.
   pr. & vb. n. Speculating.] [L. speculatus, p. p. of
   speculari to spy out, observe, fr. specula a lookout, fr.
   specere to look. See Spy.]
   1. To consider by turning a subject in the mind, and viewing
      it in its different aspects and relations; to meditate; to
      contemplate; to theorize; as, to speculate on questions in
      religion; to speculate on political events.
      [1913 Webster]

            It is remarkable that persons who speculate the most
            boldly often conform with the most pefect quietude
            to the external regulations of society. --Hawthorne.
      [1913 Webster]

   2. (Philos.) To view subjects from certain premises given or
      assumed, and infer conclusions respecting them a priori.
      [1913 Webster]

   3. (Com.) To purchase with the expectation of a contingent
      advance in value, and a consequent sale at a profit; --
      often, in a somewhat depreciative sense, of unsound or
      hazardous transactions; as, to speculate in coffee, in
      sugar, or in bank stock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

speculate
    v 1: to believe especially on uncertain or tentative grounds;
         "Scientists supposed that large dinosaurs lived in swamps"
         [syn: speculate, theorize, theorise, conjecture,
         hypothesize, hypothesise, hypothecate, suppose]
    2: talk over conjecturally, or review in an idle or casual way
       and with an element of doubt or without sufficient reason to
       reach a conclusion; "We were speculating whether the
       President had to resign after the scandal"
    3: reflect deeply on a subject; "I mulled over the events of the
       afternoon"; "philosophers have speculated on the question of
       God for thousands of years"; "The scientist must stop to
       observe and start to excogitate" [syn: chew over, think
       over, meditate, ponder, excogitate, contemplate,
       muse, reflect, mull, mull over, ruminate,
       speculate]
    4: invest at a risk; "I bought this house not because I want to
       live in it but to sell it later at a good price, so I am
       speculating" [syn: speculate, job]

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "speculate":
   be abstracted, be caught short, be short, belong, brood, bucket,
   bucketshop, buy in, buy into, call, cast a horoscope,
   cast a nativity, cast lots, cerebrate, chew over, chew the cud,
   cogitate, consider, contemplate, cut lots, cut the cards, debate,
   deliberate, deposit margin, digest, divine, dope, dope out,
   dowse for water, draw lots, draw straws, entertain a theory,
   espouse a theory, evaluate, excogitate, financier, forebode,
   forecast, foresee, foretell, fortune-tell, gamble, game, go long,
   guess, hariolate, have a theory, hold on, hypothesize, introspect,
   invest, invest in, judge, lay out money, lot, make a killing,
   make a prediction, make a prognosis, make a prophecy, make a scoop,
   make an investment, margin up, match coins, meditate,
   miss the market, mull over, muse, operate, overstay the market,
   perpend, place, play, play around with, play at dice,
   play the market, play the ponies, play with, plow back into,
   plunge, ponder, ponder over, postulate, predict, prefigure,
   presage, prognosticate, prophesy, put, pyramid, raffle off,
   read palms, read tea leaves, read the future, reason, reflect,
   reinvest, review, risk, ruminate, scalp, scoop the market,
   shoot craps, sink, sink money in, soothsay, sport, stag,
   stag the market, study, take a chance, take a flier, tell fortunes,
   tell the future, theorize, toss, toy with, trade on margin,
   vaticinate, venture, wager, weigh, wonder about