What is the plural of cactus? Well, most people would say “cacti”, using the plural form of such Latin words. But wait, is it a Latin word? Cactus is actually from the Greek kaktos, the prickly cardoon plant.(An interesting derivation, given that the Greeks never saw a cactus, which is endemic to the Americas and so the Greeks, and Romans, had to wait many hundreds of years until Columbus and his ilk got to the New World.)
Bill Casselman, in his “Canadian Word of the Day” series, speculates that the Greeks may well have misheard a word from the Egyptians. In any case, it is not a Latin word, so, as Casselman concludes, the proper plural is “cactuses.”
A search of various “authoritative” sources reveals that many assume cactus is Latin and, therefore, that the plural is cacti. Some argue that in its transition into English, the word can be its own plural, like fish. So we are left with three plurals: cactuses, cacti and cactus. Clear as mud, what?