I’ve just found this on wikipedia about Afrikaans “kos”:
From Proto-Albanian *ku̯āt-so, from Proto-Indo-European *ku̯at(h) ‘to ferment, become sour’. Compare Latin caseus (“cheese”), Latvian kûsât (“boil, simmer, seethe”), kusls (“stiff, weak”), Old Church Slavic квасъ (kvasъ, “sour dough, sour drink”), Gothic 𐍈𐌰𐌸𐍉 (ƕaþō, “scum, froth, foam”).
It seems as if “kos” and “cheese” are linked etymologically. However, that doesn’t really answer the question - how did the word make the leap to Afrikaans?
Dave, make this into another thread if you think it’s appropriate.