I think he means Mario 64 where at the start of every level Mario shouts "Mexico!"
That's "Lets A Go!" not Mexico!
I thought was Mexico too until I got a rom and emulator, not saying where, recorded that sound and then slowed it down so Mario sounded like a really old deep throated person and it came as Lets.... Ah... Go! Which sped up, at the ridiculous speed they had when they did the voice at, does sound like he says "Mexico!"
Mamma....f*****?
Erm...
1) What is the word?
2) Why would Mario swear?
3) Where you even get Mario swearing?
And to answer the question in the order asked:
1) !@#$%
2) Newgrounds.com has some blooper flash movies of Mario when something happens that appears to be a blooper and he gets mad he appears to say: "Mammaf*bleep*er"
3) He might be making a flash of some Mario bloopers that contains mature content.