Definition of "Mex"

I ran up, pulling my gun. The Mex was flat on his face, his arms stretched out. On the middle of his back knelt my one-armed friend. And that sharp hook was caught neatly under the point of the Mexican's jaw. You bet he lay still.

1907, Stewart Edward White, chapter 11, in Arizona Nights‎

[…] groups of Mex chicks swaggered around in slacks; […]

1957, Jack Kerouac, On the Road, Viking Press