The Collaborative International Dictionary of English v.0.48:

wetback \wet"back\ n.
   1. A Mexican who enters the United States illegally, by
      wading across the Rio Grande; -- disparaging and
      offensive.
      [PJC]

   2. Hence: An offensive term for a person of Mexican descent,
      usually intended and considered as an ethnic slur.
      [WordNet sense 1]

   Syn: greaser, taco.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

wetback
    n 1: (ethnic slur) offensive term for a person of Mexican
         descent [syn: greaser, wetback, taco]