“AirPods Pro” sounds stupid and, by the way, “Attorneys General” is not so hot either
tl;dr: When you have a compound noun, as opposed to a noun phrase, the proper way to pluralize it in English is just like a normal noun, with an -s at the end, not jammed in the middle, attached to what is now just another morpheme. But whether due to hypercorrection, or the legacy of noun phrases that have become compound nouns, sometimes you stick the -s in the middle anyway. But this is better thought of as an exception–an irregular plural–rather than a “correct” application of the standard plural rule to a noun that happens to have a postpositive adjective.
Apple has called its new wireless headphones “AirPods Pro”–not “AirPod Pros” or “Pro AirPods.” Presumably just the right or left headphone would be just a singular “AirPod Pro.” A lot of people, and probably Apple too, think that this is the “correct” way to pluralize in this situation.
They are wrong. Well, sort of. At least, not so clearly right. “AirPod Pro” is a single word and, in English, you don’t pluralize words by sticking an “-s” in the middle. You stick an -s at the end. I mean, you can do it if you want–no one’s going to arrest you–but in so doing you’ve come up with a new way for pluralizing certain nouns that is always going to sound a bit weird and affected, and goes against the grain of how English normally works. At best you’ve coined a new irregular plural.
The obvious objection is that “AirPod” is the noun, and “Pro” is just an adjective that for weird reasons is “postpositive,” so you’d say “AirPods Pro” for the same reason you’d say “pro gamblers.” But it’s not that simple.
The English language is full of compound words. Sometimes they are spelled as if they are two words (like “White House” and “high school”) because once they were two words, and spelling tends to preserve old forms of speech. If you build a market and you think it’s really super, you’ll probably call it a “super MARKET,” with more stress on “market.” That shows you that, at this point in the evolution of “super market,” it’s still a phrase, and likely refers to something quite novel.
But as the phrase becomes more common, it will begin to be pronounced with the usual stress pattern you see in multisyllable words in English–as “SUPERmarket,” which is in fact how it’s pronounced today. This follows the same pattern as other English nouns such as “chicken” and “bullshit,” where the stress is at the beginning. “High school” is an even better illustration (thanks to [redacted] for this example), where the stress on the initial syllable has actually changed the vowel quality of “high” from the “long i” sound in “eye” to the (phonetically distinct) “long i” sound in “might.”
Once the ceaselessly changing patterns of speech have turned a phrase into a compound word, it doesn’t matter where the individual units of meaning (now better referred to as “morphemes,” not “words”) came from. The word “dog” has just one morpheme: dog. Dogs has two, dog and -s. And “dog houses” has three: dog, houses, and -s (pronounced “izz”). In English, “cul-de-sac” is just a single word with a single morpheme. It doesn’t matter that “cul,” “de,” and “sac” are words in French, as none of them carry any independent meaning in English.
Similarly, it doesn’t matter what grammatical role the various sounds once played in a foreign language. “Lieutenant General” and “Attorney General” also both come from French. In French, “general” is the adjective, and “attorney” and “lieutenant” (spelled and pronounced differently, and borrowed from Old French, not modern French) are nouns. And in French you put adjectives after nouns. But for no particular reason in English we decided that a “General” would be noun referring to a category of military officer, even though it originally was just an adjective (“general” officers have more general duties than field officers directly in charge of troops). But probably because we also adopted the word “attorney” separately, in “Attorney General,” “Attorney” was still seen as the “noun,” with the adjective “general” just anomalously put after it, as in French, instead of the more Englishy way of doing things, which would be “General Attorney.”
But whatever its origins, “Attorney General” is now a single noun, not a phrase. We know this just by looking at how it is pronounced in everyday speech. Like with other compound words, the stress has shifted to the first part of the word–John McWhorter calls this the “backshift” in his book Words on the Move. It’s the supermarket thing, mentioned above, or the difference between “A crow is a black bird” and “Look at the blackbird.” Plus, while there are a few random adjectives that can only be used postpositively, such as “aplenty,” “galore” and “akimbo,” “general” isn’t one of them. If “Attorney General” were a noun+adjective phrase, rather than a compound noun, you’d be able to change it around and say “General Attorney” and be understood, just as “lost paradise” means the same thing as “paradise lost.” But you can’t.
Thus the normal way to pluralize “Attorney General” in English should be “Attorney Generals,” not “Attorneys General.” This is by contrast to phrases that happen to have postpositional adjectives but are not treated in speech as compound noun: You would certainly say “Queens regnant,” “Presidents-elect,” or “demons incarnate.” But there’s no reason to think that you should pluralize compound nouns differently depending on the etymological origin of their various components.
That said, depending on your social context, you’re better off sticking with the unnatural “Attorneys General” because otherwise people will think you’re making a mistake. And maybe you are! But for a different reason than misapplying the normal -s plural rule. In the corpus of printed works “Attorneys General” is much more common than “Attorney Generals,” but it’s better thought of as an irregular plural, than an application of the normal rules of English grammar. Viewed this way, “Attorneys General” not “wrong,” but it is an exception, like “mice” or “beeves.” However, it weren’t for social considerations and hypercorrection, maybe “Attorneys General” would sound as unnatural as “sits up” as the plural of sit ups.
Thought experiment: let’s say we started talking about this demon incarnate, that demon incarnate, all day every day. Maybe there’s no other kind of demon, and no other way of things becoming incarnate. Would it ever switch to “demon incarnates”? In a purely spoken language, maybe, but the written word has a pull on people and can prevent linguistic change that would otherwise happen. In any event irregular plurals in general are usually the fossilized remnants of old rules that used to be more systematic, but eventually just have to be learned by speakers on a one-off basis. So children learning to speak might learn “demons incarnate” as an exception, just as children today learn “children” as an exception, as opposed to the remnants of an older rule about -en plurals that now survives in just children, oxen, and brethren.
Back to “AirPods Pro.” When products are given names like “Whopper Junior,” “iPhone 11,” “MacBook Pro,” and “AirPods [sic] Pro,” they tend to be analyzed–and pronounced–as compound nouns, not as phrases. So the plural should go at the end: Whopper Juniors, MacBook Pros, iPhone 11s. Apple’s new product would much more naturally be called “Pro AirPods” or “AirPod Pros,” unless, as seems to have happened with “Attorneys General,” Apple wants to force a new irregular plural onto the scene. (You might argue that compound phrases only become single words slowly, and that “AirPod(s) Pro” simply hasn’t yet. But I think it is so closely modeled after “MacBook Pro,” an established compound noun, that it starts out as one.)
Anyway, I’d have gone with “Pro AirPods.”