The Collaborative International Dictionary of English v.0.48:

Dupe \Dupe\ (d[=u]p), n. [F., prob. from Prov. F. dupe, dube; of
   unknown origin; equiv. to F. huppe hoopoe, a foolish bird,
   easily caught. Cf. Armor. houp['e]rik hoopoe, a man easily
   deceived. Cf. also Gull, Booby.]
   One who has been deceived or who is easily deceived; a gull;
   as, the dupe of a schemer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dupe \Dupe\, v. t. [imp. & p. p. Duped (d[=u]pt); p. pr. & vb.
   n. Duping.] [Cf. F. duper, fr. dupe. See Dupe, n.]
   To deceive; to trick; to mislead by imposing on one's
   credulity; to gull; as, dupe one by flattery.
   [1913 Webster]

         Ne'er have I duped him with base counterfeits. --
                                                  Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dupe
    n 1: a person who is tricked or swindled [syn: victim, dupe]
    v 1: fool or hoax; "The immigrant was duped because he trusted
         everyone"; "You can't fool me!" [syn: gull, dupe,
         slang, befool, cod, fool, put on, take in, put
         one over, put one across]