The Collaborative International Dictionary of English v.0.48:

Inquisitor \In*quis"i*tor\, n. [L.: cf. F. inquisiteur. See
   Inquire.]
   1. An inquisitive person; one fond of asking questions. [R.]
      "Inquisitors are tatlers." --Feltham.
      [1913 Webster]

   2. (Law) One whose official duty it is to examine and
      inquire, as coroners, sheriffs, etc. --Mozley & W.
      [1913 Webster]

   3. (R.C.Ch.) A member of the Court of Inquisition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inquisitor
    n 1: a questioner who is excessively harsh [syn: inquisitor,
         interrogator]
    2: an official of the ecclesiastical court of the Inquisition

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "inquisitor":
   Paul Pry, Peeping Tom, asker, busybody, catechist,
   cross-interrogator, cross-questioner, detective, eavesdropper,
   examiner, gossip, inquirer, inquisitionist, inquisitive,
   interlocutor, interpellator, interrogator, interrogatrix,
   interviewer, nosy Parker, opinion-sampler, poller, pollster,
   prober, pry, querier, querist, questioner, questionist, quidnunc,
   quizzer, rubberneck, rubbernecker, sampler, scopophiliac,
   secret agent, sightseer, snoop, snooper, voyeur, yenta



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INQUISITOR. A designation of sheriffs, coroners, super visum corporis, and
the like, who have power to inquire into certain matters.
     2. The name, of an officer, among ecclesiastics, who is authorized to
inquire into heresies, and the like, and to punish them. An ecclesiastical
judge.