That word did not originate as a racial slur, but became one in 20th-century South Africa, according to Wikipedia.
In Arabic, the word kāfir ("unbeliever") was originally applied to non-Muslims before becoming predominantly focused on pagan zanj (black African) who were increasingly used as slaves.[2] During the Age of Exploration in early modern Europe, variants of the Latin term cafer (pl. cafri) were adopted in reference to non-Muslim Bantu peoples even when they were monotheistic. It was eventually used, particularly in Afrikaans (Afrikaans: kaffer), for any black person during the Apartheid and Post-Apartheid eras, closely associated with South African racism, it became a pejorative by the mid-20th century and is now considered extremely offensive