The Collaborative International Dictionary of English v.0.48:

jinx \jinx\ (j[i^][ng]ks), n.
   A person, object, influence, or supernatural being which is
   supposed to bring bad luck or to cause things to go wrong.
   [Slang]
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

jinx \jinx\ (j[i^][ng]ks), v. t.
   To bring bad luck to; to cause to malfunction or go wrong;
   as, some superstitious people are reluctant to predict
   success for fear it will jinx them. [Slang]
   [PJC]

WordNet (r) 3.0 (2006):

jinx
    n 1: a person believed to bring bad luck to those around him
         [syn: jonah, jinx]
    2: an evil spell; "a witch put a curse on his whole family"; "he
       put the whammy on me" [syn: hex, jinx, curse, whammy]
    v 1: cast a spell over someone or something; put a hex on
         someone or something [syn: hex, bewitch, glamour,
         witch, enchant, jinx]
    2: foredoom to failure; "This project is jinxed!"