Idk but i THINK its because some of them have dark hair that looks greasy because its slicked back a lot... It could be because of the food they eat though so idk. I mean all races get stereotyped and have weird names so its nothing to get offended by.
Greasers were a southern U.S. gang-related culture in the 50's having affiliations with street rods and rockabilly music. Some Mexicans may have adopted the culture... but it wasnt particular or attributable to Mexicans.
It wasnt just mexicans but anyways its because back in the 50s and 60s the lower social group had slicked their hair back and the higher group just called them that hope this helped! :)
I'm not offend i was just wondering cause i was talking to my dad and he was saying in school they use to call him a greaser when he was young.