Plurals of loanwords of obviously foreign form

What rules of thumb govern when to pluralise a foreign word as it should be in the original language and when it should be pluralised as an English word?

For example, you'd get some funny looks using "octopodes" or "lemmata" in normal conversation. (I don't know what contexts would allow mention of cephalopods and mathematics, but you know...)

Solution 1:

General rule of thumb is that words which obviously originate from Latin and are not in frequent use might be given the Latin plural (e.g. alumnus/alumni with 22m Google hits). More common Latin words usually have an English plural (e.g. campus/campuses 122m; bonus/bonuses 108m).

For languages other than Latin (and perhaps French), English plural is almost always used, even for rare words (e.g. klutz 5m), except where plural is the predominant or only form (e.g. spaghetti).

This is completely unscientific, my intuition as a native speaker (American). Counterexamples are welcome.

Solution 2:

It doesn't seem to me there are any rules of thumb for how to pluralize a foreign word.

The New Oxford American Dictionary reports that both lemmata, and lemmas are the plural of lemma. If I check on the Corpus of Contemporary American English, and I search for lemma*, I notice that

• The word is used mostly in academic contexts.
• In an academic context, the word lemmata is never used; in one case, the word used is lemmae.

  Other examples of lemmae held to be doublets, and furnished with catchwords, but which actually occur three […].

If then I search for lemma* in the Corpus of Historical American English, I notice that the word lemmata is used for the first time in the 1900s; for the other years, before or after, the CoHA doesn't report the word lemmata.

For octopus the NOAD reports that

The standard English plural of octopus is octopuses. However, the word octopus comes from Greek, and the Greek plural form is octopodes. Modern usage of octopodes is so infrequent that many people mistakenly create the erroneous plural form octopi, formed according to rules for Latin plurals.

Looking for octop* in the CoCA, I found out that

• Words starting with octop are used with higher frequency in fiction and magazines.
• In magazines, where words starting with octop are used with an incidence of 3.14 per million, I find only three examples of text using _octopodi; in the other cases, the used words are octopus and octopuses. I also found the word octopusan, and Octopussy (the title of a James Bond movie).

  If such a quality as intelligence can arise both in human beings and in the octopusan eight-armed sea animal without a bone in its body, then perhaps there is a course and […].

A search for octop* in the CoHA gives the following results:

• Octopodi is used through the years with a variable incidence.

• Octopodes is used in one case.

  In addition to being unspellable (octopuses, octopusses, octopoi, octopi, octopodes?), it conjured up every old horror cliche.

• There are a few instances of words like octopoid, and octopodous.

  Barney said, struggling free of the perfumed octopoid embrace, "just take your client aside for a moment then I'll be […]".
  […] Philharmonic Society, which, as I have already pointed out, is within the octopodous grip of the BBC even to the extent of having to satisfy that totalitarian body […].