Kaffir, a term that comes from the Arabic word kafir, originally meaning "non-believer" or "infidel", was first used by Arabic-speaking Muslims to refer to the polytheist Black people of Central and South Africa and Southern India. It passed to some European languages to denote the local peoples of these areas, and by extension, everything that came from South Africa and southern India and Sri Lanka.
The word has been used in the vernacular names of several edible fruits associated with these regions: Kaffir apple (Dovyalis caffra), Kaffir plum (Harpephyllum caffrum), both from South Africa, and Kaffir lime (Citrus hystrix), due to the fact this citrus was commonly grown in Sri Lanka.
In recent decades, the word Kaffir, in South Africa, has been increasingly perceived as an ethnic slur, and its use to demean black persons in that country is now regarded as politically incorrect. Though in the fruit names, the term has only a geographic meaning, and despite the fact that, in the case of Kaffir lime it referred to Sri Lanka rather than to Africa, some have also called for its ban and replacement by synonymous, advocating for instance the use of "Thai Lime" or "Makrut" as a replacement name for the Kaffir lime.
However, Kaffir lime remains the most commonly used English word used to name the fruit of Citrus hystrix.