“Kaffir” is thought to ultimately derive from the Arabic kafir, meaning infidel, though the mechanism by which it came to be applied to the lime is uncertain. Following the takeover of the Swahili coast, Muslims used the term to refer to the non-Muslim indigenous Africans, who were increasingly abducted for the Indian Ocean slave trade, which reached a height in the fifteenth and sixteenth century.
The most likely etymology is through the Kaffirs, an ethnic group in Sri Lanka partly descended from Bantu slaves. The earliest known reference, under the alternative spelling “caffre” is in 1888 book The Cultivated Oranges, Lemons Etc. of India and Ceylon by Emanuel Bonavia, who notes, “The plantation coolies also smear it over their feet and legs, to keep off land leeches; and therefore in Ceylon [Sri Lanka] it has got also the name of Kudalu dchi, or Leech Lime. Europeans call it Caffre Lime.” Similarly, H.F. MacMillan’s 1910 book A Handbook of Tropical Gardening and Planting notes, “The ‘Kaffir Lime’ in Ceylon.”
Another proposed etymology is directly by Indian Muslims of the imported fruit from the non-Muslim lands to the east to “convey otherness and exotic provenance.” Claims that the name of the fruit derives directly from the South African ethnic slur “kaffir” (see “Name” below) are not well supported.
C. hystrix is known by various names in its native areas:
jeruk purut in Indonesian and limau purut in Malay. Purut, “rough-skinned”, refers to the bumpy texture of the fruit.
jiàn yè chéng (箭叶橙) in Chinese.
kabuyaw or kulubot in the Philippines. The city of Cabuyao in Laguna is named after the fruit.
Kolumichai, கொலுமிச்சை in Kongu Tamil
makrud or makrut (มะกรูด, /máʔ.krùːt/) in Thailand (a name also used for the bergamot orange).
mak khi hut (ໝາກຂີ້ຫູດ, /ma᷆ːk.kʰi᷆ː.hu᷆ːt/) in Laos.
trúc or chanh sác in Vietnam.
combava in Réunion Island
The micrantha, a similar citrus fruit native to the Philippines that is ancestral to several hybrid limes, such as the Key lime and Persian lime, may represent the same species as C. hystrix, but genomic characterization of the kaffir lime has not been performed in sufficient detail to allow a definitive conclusion.
source : https://en.wikipedia.org/wiki/Kaffir_lime