Why when people try to be raciest against hispanic people they call them wetbacks?

Im african american but my family have a big ethnicity background we all are christians but we do things differently.my favorite girl cousin is mexican and we grew up in gary and chicago where race can get you killed.she use to tell me about people being racist i was her age i got into some fights here or there but now her lil sis is going through it and people call her a setback i honestly never used the word and i also don't know what it means i know its a racist word and inever justify racist words i even stopped using the n word so i would stop being a hypocrite (1yr strong)but i never was told by my hispanic part or any other part of my family or friends what it means so i wanna know the meaning and why you shouldn't call a person that other than its racist i will give 5 star to who ever give me a good explanation thanks.