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]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "courier":
   Gyropilot, Hermes, Iris, Mercury, Paul Revere, Pheidippides,
   automatic pilot, bearer, boatheader, boatsteerer, carrier,
   cicerone, commissionaire, cowherd, coxswain, diplomatic courier,
   dragoman, drover, emissary, envoy, estafette, express, go-between,
   goatherd, guide, guidepost, guider, helmsman, herd, herdsman,
   internuncio, mercury, message-bearer, messenger, navigator, nuncio,
   pilot, pointer, post, postboy, postrider, river pilot, runner,
   shepherd, steerer, steersman, tour director, tour guide



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.


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

COURIER. One who is sent on some public occasion as an express, to bear
despatches, letters, and other papers.
     2. Couriers sent by an ambassador or other public minister, are
protected from arrest or molestation. Vattel, liv. 4, c. 9, Sec. 123.