The Collaborative International Dictionary of English v.0.48:

Conspire \Con*spire"\, v. t.
   To plot; to plan; to combine for.
   [1913 Webster]

         Angry clouds conspire your overthrow.    --Bp. Hall.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Conspire \Con*spire"\ (k[o^]n*sp[imac]r"), v. i. [imp. & p. p.
   Conspired (k[o^]n*sp[imac]rd"); p. pr. & vb. n.
   Conspiring.] [F. conspirer, L. conspirare to blow together,
   harmonize, agree, plot; con- + spirare to breathe, blow. See
   Spirit.]
   1. To make an agreement, esp. a secret agreement, to do some
      act, as to commit treason or a crime, or to do some
      unlawful deed; to plot together.
      [1913 Webster]

            They conspired against [Joseph] to slay him. --Gen.
                                                  xxxvii. 18.
      [1913 Webster]

            You have conspired against our royal person,
            Joined with an enemy proclaimed.      --Shak.
      [1913 Webster]

   2. To concur to one end; to agree.
      [1913 Webster]

            The press, the pulpit, and the stage
            Conspire to censure and expose our age. --Roscommon.

   Syn: To unite; concur; complot; confederate; league.
        [1913 Webster]

WordNet (r) 3.0 (2006):

conspire
    v 1: engage in plotting or enter into a conspiracy, swear
         together; "They conspired to overthrow the government"
         [syn: conspire, cabal, complot, conjure,
         machinate]
    2: act in unison or agreement and in secret towards a deceitful
       or illegal purpose; "The two companies conspired to cause the
       value of the stock to fall" [syn: conspire, collude]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "conspire":
   accord, act in concert, act together, affiliate, agree, ally,
   amalgamate, angle, associate, band, band together, be in cahoots,
   be in league, brew, bunch, bunch up, cabal, cement a union,
   centralize, club, club together, coact, coadunate, coalesce,
   cogitate, coincide, collaborate, collogue, collude, combine,
   come together, complot, concert, concoct, concord, concur,
   confederate, conjoin, connive, consociate, consolidate, contrive,
   cook up, cooperate, correspond, countermine, counterplot, couple,
   devise, do business with, engineer, federalize, federate, finagle,
   finesse, frame, frame up, fuse, gang, gang up, get heads together,
   get together, go in partners, go in partnership, go partners,
   hang together, happen together, harmonize, hatch, hatch a plot,
   hatch up, hold together, hook up, hook up with, intrigue, join,
   join forces, join fortunes with, join in, join together,
   join up with, join with, keep together, lay a plot, league,
   league together, machinate, make common cause, maneuver, marry,
   merge, operate, organize, pair, pair off, partner, play ball, plot,
   pull together, put heads together, reciprocate, rig, scheme,
   stand together, stand up with, synchronize, synergize, team up,
   team up with, team with, throw in together, throw in with, tie in,
   tie in with, tie up, tie up with, unionize, unite, unite efforts,
   unite with, wangle, wed, work together