The Collaborative International Dictionary of English v.0.48:

Greaser \Greas"er\, n.
   1. One who, or that which, greases; specifically, a person
      employed to lubricate the working parts of machinery,
      engines, carriages, etc.
      [1913 Webster]

   2. A nickname sometimes applied in contempt to a Mexican or
      other Latin-American of the lowest type; -- derogatory and
      offensive. [Low, U. S.]
      [1913 Webster +PJC] grease gun

WordNet (r) 3.0 (2006):

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