I know that in this useage it does not mean Mexico or Mexican. Nor have Iyou mitaken it for “max.” I saw it once in print – “two seconds mex.” With the apparent meaning of very fast. I have also heard it used in the phrase “There is a lot of money there. 6 million mex!” From the movie “The Bitter Tea of General Yen.” The apparent meaning there was to impress with the size of the amount. It was said very clearly, so there was no chance of mistaking it for “max,” which would not have fit in any case.

It seems to some kind of slang intensifier.

I certainly hope someone can help. I have checked many standard and slang dictionaries both in print and on line with no sensible result.