Meximierda is disparaging word commonly used by a Latin person to insult a Mexican. Latins for the most part view themselves above Mexicans and that's the main reason why they called Mexicans Meximierdas.
Meximierda in English means Mexi-shit. Meximierda is a slang word for Mexican in Latin circles. A Latin views a Mexican as inferior and worthless for the most part.
Ramon: None of the girls in the club want to dance with the ugly Meximierda Mexican.
Fernando: The girls can smell the shit of the Meximerda Mexican a mile away.