Mexicaca is a derogatory term that is used when someone talks about a Mexican. It is usually used by a non Mexican Latino but other races have been known to use the term as well. The literal translation of Mexicaca is Mexi-Shit as caca means shit in Spanish.
The reason a Latino would normally use the word is because of US culture grouping every Latino as a Mexican. Thus hate grew towards the Mexican because they feel Mexico represents nothing of them and their home country. Others feel when they are called "Mexican" they are seen with the usual stereotypes of a Mexican.
John: "Hey Juan, where are you from?"
John: "What part of Mexico is that?"
Juan: "Shut your racist shit up! I'm not a Mexicaca!"
Pedro: "Mira ese Mexicaca." "El piensa que es la gran mierda!"
translation: Look at that Mexicaca. He thinks he's the shit!
Pablo: "Simon!" "Sucio culero." "Por eso odio esos Mexicacas!"
translation: Yeah man! Dirty faggot. That's why I hate those Mexicacas!
Mexicaca is a racially motivated insult that originated in the late '70s against any Mexican walking down the street. Mexicaca is a cruel word to insult a Mexican at a party, school or work. Latins who hate Mexicans claim that Mexicans smell like shit and that's the reason why they call them Mexicacas.
Mario: Look at those dwarfs Mexicacas singing the Mexican anthem. ha ha
Javier: Mexicacas don't look like human beings, dude, they look like they're dwarfs from another planet. ha ha