It's hard not to be in awe of an octopus. Not only is it one of Earth's smartest invertebrates, but it seems like it's from another planet. It has psychedelic skin, shape-shifting skills and eight arms that hold two-thirds of its neurons. A wild octopus uses its diffuse, alien brain to find prey and evade predators. In captivity, it wows humans by solving mazes, using tools, escaping tanks and taking photos of us.
One of the octopus's most vexing mysteries, however, is more about etymology than biology. The animal may be one in a million, but what do we call two or more of them? Are they "octopuses"? Are they "octopi"? Or is there another, even more esoteric word that's technically the most correct?
Yes, yes and yes. Nothing is ever simple with octopodes.
"Octopi" is a commonly used plural, and it seems to make sense. After all, similar words that end in -us are pluralized with an -i ending, like foci, loci or alumni. But while focus, locus and alumnus are Latin words, octopus hails from ancient Greek.
As the Grammarist explains, octopi "has no etymological basis." It only exists due to a modern fallacy that octopus comes from Latin. Its actual origin is the Greek word oktopous, which literally means "eight-footed." The -us in octopus is thus a relic of the Greek pous for "foot," not the second-declension masculine Latin ending whose plural form is -i. That means the correct plural is octopodes, but as the Online Etymological Dictionary adds, "octopuses probably works better in English."
More than meets the 'i'
It's worth noting that octopus is a Latinized Greek word, although it comes to English via New Latin, aka scientific Latin, not the language of ancient Rome. The first known use of the word was in 1758.
It's also worth noting that English uses lots of words from Latin and from newer languages, often without preserving their original plurals. In Latin, for example, the correct plural of "circus" would be circi. So even if octopus was a true Latin word, we'd be under no obligation in 2015 to say octopi. Most dictionaries include the Anglicized plurals "focuses" and "terminuses" as alternatives to foci and termini, and many also now allow octopi as a secondary plural in lieu of octopuses or octopodes.
At least the octopus isn't alone in this linguistic ambiguity. The rhinoceros, hippopotamus and platypus are all in the same boat, stuck with Latinized Greek names and disputed plurals. In Greek, rhinokeros means "nose-horned," hippopotamos means "river horse" and platypous means "flat-footed." Their preferred English plurals are rhinoceroses, hippopotamuses and platypuses, but Merriam-Webster dictionary also lists alternative -i plural forms for all three.
Octopi is still based on a misconception, Oxford Dictionaries points out, and it remains less common in edited writing than octopuses. But that doesn't mean it's wrong — in fact, it highlights an important point about words in general. Language is a fluid, crowd-sourced reflection of its human creators, so any word is correct if enough people use and understand it (yes, even an abomination like "irregardless").
Plus, the more time we spend arguing about semantics, the less time we'll have to prepare for the inevitable overthrow of civilization by superintelligent octopi. I mean octopodes.