Those Dirty Mexicans--Oh, Really?

When my wife and I announced that we were moving to Mexico, one of my wife's relatives asked us,

"Why do you want to live with those dirty Mexicans?"

Besides wanting to slap some sense into this person, I was particularly sickened (as in wanting to projectile vomit Linda Blair-style) by this hideous and most heinous stereotype. I have wanted to know since then where this originated.

I think, after doing what writers do