[jjtheblunt]

octopi arises because people have learned Latin 2nd declension masculine nouns, by accident or repetition, where pluralizing (nominative case) turns the -us to -i.

It's a pattern matching phenomenon.

It would be less weird if there were not spelling irregularities, since the -pus is meant to be foot (like pes, pedes...in Latin, or pos, podes,... in Greek).

Basically, the spelling irregularity triggers a sensible pattern match, which happens to of course not honor the spelling irregularity.

And then nerds like me write too much about such, but i had years of Latin (and a little Greek) for something!

[jacobmartin]

Maddeningly, it doesn't even apply to all Latin -us nouns. For instance, the plural of apparatus should be apparatūs if we're applying the Latin rules. Apparati, as it might seem, makes no sense! So even when the pattern match correctly identifies the language, it can be misleading.

--fellow Latin nerd

[jjtheblunt]

apparatus is a past participle from apparare, so as an adjective plural would follow second declension, not fourth, i thought.

used as a noun, it's an (implicit thing) prepared (which preparatus would describe, but i guess we don't have any other word in English other than apparatus.

i don't have a Latin dictionary though

[jacobmartin]

apparatus is first/second as a past participle. I suppose it could be used as a substantive, but there is also a fourth declension noun with the same lemma meaning implements, tools, etc.: https://logeion.uchicago.edu/apparatus

[jjtheblunt]

4th or 5th declension esoterica ftw!

[daveslash]

Ironically, when not used as a word ending, but as a single word... "I" is a singular way to talk about a person (namely, oneself), where as "us" is a plural way to talk about multiple people.