different between octopii vs octopi

English

Etymology

In imitation of Latin nominative plural forms of nouns ending in ius. Compare walrii and penii.

Noun

octopii

1. (nonstandard, proscribed, rare) Misconstructed plural form of octopus.

Usage notes

• The plural form *octopii is doubly incorrect. Firstly, octopus derives from Greek, not Latin; its etymologically-consistent plural form is octopodes. Secondly, even if octopus were a second-declension Latin noun, the plural form would be *octopi; in the correct plurals radii and gladii, with which *octopii is analogous, the first ‘i’s are part of the words’ stems (radi- and gladi-), and not their case endings — for *octopii to be the plural, *octopius would need to be the singular.

English

Etymology

Created in Latin by analogy with p?lypus (“octopus”), which was either second or third declension in both Latin and Greek, though historically third in the latter.

Noun

octopi

1. (hypercorrect) plural of octopus

Usage notes

The forms octopuses and octopi appear roughly equally prevalent, with octopodes being much rarer. However, octopi is hypercorrect. See octopus for further discussion.