This is a
pejorative term
A pejorative or slur is a word or grammatical form expressing a negative or a disrespectful connotation, a low opinion, or a lack of respect toward someone or something. It is also used to express criticism, hostility, or disregard.
› wiki › Pejorative
What does the Spanish word Rinche mean?
rinche. adjective. (And) (S. Cone) full to the brim; brimming over.
What does Rinche mean in From Dusk Till Dawn?
A quick google search highlights that "rinche" is "A pejorative term used for Texas Rangers in Mexico. Border Guard. Literally brimming over." (see here).