Answer:
   
   Shiksa and Shaygetz are the Yiddish derivative of the respective
   feminine and masculine Hebrew words for something unclean, dirty. The
   appellations are customarily applied to gentiles who do things
   inimical to Jewish interests, such as vandalizing Jewish buildings,
   robbing Jewish kids of their lunch money, or becoming romantically
   involved with Jews :-). The root is "sheketz", which refers to house
   rodents and lizards. They impart ritual impurity, and therefore the
   term lends itself to the same kind of idea. Some have taken to using
   the term to refer to Christian women in general. If Christians were
   using the term against Jews in English, they would be saying "Filthy
   Jews" or "Dirty Jews", and we Jews would rightly be offended. Hence,
   use of these terms should really be avoided; it is insulting and
   inappropriate, even if no bad intent was behind the usage. It is
   always better to use neutral, less pejorative (judgemental) terms,
   such as non-Jew or Christian.
   
   Note: In Israel, shaygetz is sometimes used to refer to a misbehaving
   child.
   
   Note: There are other words for non-Jewish women, "nachriah", and
   "goyah", that are more properly used in less judgemental situations.