The Collaborative International Dictionary of English v.0.48:

Trojan \Tro"jan\, a. [L. Trojanus, fr. Troja, Troia, Troy, from
   Tros, Gr. Trw`s, Trwo`s, Tros, the mythical founder of Troy.]
   1. Of or pertaining to ancient Troy or its inhabitants. -- n.
      A native or inhabitant of Troy.
      [1913 Webster]

   2. One who shows the pluck, endurance, determined energy,
      strength, or the like, attributed to the defenders of
      Troy; -- used chiefly or only in the phrase

   like a Trojan; as, he endured the pain like a Trojan; he
      studies like a Trojan.
      [Webster 1913 Suppl.]

            Tim jumped like a Trojan from the bed. --Finnegan's
                                                  Wake (Irish
                                                  song)
      [PJC]

WordNet (r) 3.0 (2006):

Trojan
    adj 1: of or relating to the ancient city of Troy or its
           inhabitants; "Trojan cities"
    n 1: a native of ancient Troy [syn: Trojan, Dardan,
         Dardanian]
    2: a program that appears desirable but actually contains
       something harmful; "the contents of a trojan can be a virus
       or a worm"; "when he downloaded the free game it turned out
       to be a trojan horse" [syn: trojan, trojan horse]

The Free On-line Dictionary of Computing (30 December 2018):

Trojan horse
trojan

    (Or just "trojan") A term coined by
   MIT-hacker-turned-NSA-spook Dan Edwards for a malicious,
   security-breaking program that is disguised as something
   benign, such as a directory lister, archiver, game or (in one
   notorious 1990 case on the Mac) a program to find and destroy
   viruses!  A Trojan horse is similar to a back door.

   See also RFC 1135, worm, phage, mockingbird.

   [Jargon File]

   (2008-06-19)