The Collaborative International Dictionary of English v.0.48:

dissociate \dis*so"ci*ate\ (d[i^]s*s[=o]"sh[i^]*[=a]t), v. t.
   [imp. & p. p. Dissociated; p. pr. & vb. n. Dissociating.]
   [L. dissociatus, p. p. of dissociare to dissociate; dis- +
   sociare to unite, associate, socius companion. See Social.]
   To separate from fellowship or union; to disunite; to
   disjoin; as, to dissociate the particles of a concrete
   substance.
   [1913 Webster]

         Before Wyclif's death in 1384, John of Gaunt had openly
         dissociated himself from the reformer.   --A. W. Ward.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dissociate
    v 1: part; cease or break association with; "She disassociated
         herself from the organization when she found out the
         identity of the president" [syn: disassociate,
         dissociate, divorce, disunite, disjoint]
    2: regard as unconnected; "you must dissociate these two
       events!"; "decouple our foreign policy from ideology" [syn:
       decouple, dissociate] [ant: associate, colligate,
       connect, link, link up, relate, tie in]
    3: to undergo a reversible or temporary breakdown of a molecule
       into simpler molecules or atoms; "acids dissociate to give
       hydrogen ions"