Puro Mexicano

You see I have been reading bogs quite a bit and talking with some people in my work, and some of them have used the term "puro mexicano" to descibe themselves and I am not sure I understand. I would imagine that to each it means something a little different, but each time it seems to attempt to say that they are one with their culture. Some I have noticed use the term to fault others for what they deem a lack of retained culture. But what does the term mean? I mean if you are a Citizen of the Republic of Mexico the meaning is clear, but what if you are or you wish to become a Citizen of the United States?