Guancaca is a slur used to describe Salvadorans in a derogatory way. Probably from "guanaca," which is non-derogatory Central American slang for a Salvadoran + "caca," which means what it means in the US
The word Guancaca is a racial slur that has been used historically to degrade and dehumanize salvadorans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to salvadorans. If you are unsure about whether or not it is appropriate to use the word Guancaca, it is best to err on the side of caution and avoid using it in any situation.