The Collaborative International Dictionary of English v.0.48:

Courier \Cou"ri*er\ (k??"r?-?r), n. [F. courrier, fr. courre,
   courir, to run, L. currere. See Course, Current.]
   1. A messenger sent with haste to convey letters or
      dispatches, usually on public business.
      [1913 Webster]

            The wary Bassa . . . by speedy couriers, advertised
            Solyman of the enemy's purpose.       --Knolles.
      [1913 Webster]

   2. An attendant on travelers, whose business it is to make
      arrangements for their convenience at hotels and on the
      way.
      [1913 Webster]

WordNet (r) 3.0 (2006):

courier
    n 1: a person who carries a message [syn: messenger,
         courier]

The Jargon File (version 4.4.7, 29 Dec 2003):

courier


    [BBS & cracker cultures] A person who distributes newly cracked warez, as
    opposed to a server who makes them available for download or a leech
    who merely downloads them. Hackers recognize this term but don't use it
    themselves, as the act is not part of their culture. See also warez d00dz
    , cracker, elite.