I was listening to the radio the other day and I heard the host of a classical music program (during the program, not in a promo) say, unprompted, “imprompti” as a plural for impromptu.
What! Imprompti? Pre-empting the familiar impromptus? I promptly looked it up to make sure I was not mistaken. But I was not: the English plural is impromptus. Some sources say we got it via French, wherein the plural is impromptus. But impromptu ultimately comes from Latin, where it is in promptu (Latin did not implement the /n/-to-/m/ place assimilation before /p/ in this instance, interestingly; it was left to French and English to do that). And the plural of Latin in promptu is…
Well, before I tell you, I need to give you a bit of background and context. Otherwise it won’t make enough sense. Let’s start, briefly, with that in. Often when we see in- or im- as a prefix in words from Latin, it’s a negator: immovable, indecent. Other times it’s an intensifier: invaluable. Sometimes it’s an intensifier later mistaken as a negator: inflammable, infamous. But sometimes it just means ‘in’, as in the same kind of thing as English in. Like in insert. And that’s what it is here – impromptu does not mean ‘not promptu’, nor does it mean ‘very promptu’; it means ‘in promptus’ (we will get to the question of promptus becoming promptu shortly). And what is promptus?
You might recognize prompt there, and you’ll be right if you do. Promptus is the source of that. As a noun, promptus means ‘readiness’ or ‘an exposing to view’; the noun is formed from the past participle of promo – which, amusingly, is not related to English promotion. No, this is pro ‘for, forward’ plus emo ‘I buy, I take’ (and no relation to emotion – sorry!). You may know the Latin phrase caveat emptor, ‘buyer beware’; that emptor is from the same root. And the past participle of emo is emptus, as in pre-empt (but not empty, which is a Germanic word).
So pro plus emo makes promo ‘I bring forth’, which formed promptus ‘brought forth’, which makes the noun promptus ‘readiness’ or ‘exposing to view’. So the plural of promptus is prompti, right? Ha ha, no. Latin is less simple than we might want; if given the chance to learn all the Latin word forms, we might want to decline, but they already have declined, and this is a fourth declension noun, which forms its forms differently. Let me add the scholarly macrons to mark length (not written in Latin of the time but useful to us today to distinguish vowel length): singular prōmptus in the nominative becomes plural prōmptūs, and in the accusative it’s singular prōmptum and plural prōmptūs. But, as noted, it’s in promptu and not in promptus. That’s because in in this case governs the ablative case (ablative comes from Latin for ‘taking away’, probably because many people wish it would be taken away). And singular in prōmptū pluralizes to in prōmptibus.
But that really doesn’t matter, because when we’re talking about several instances of improvisations (an impromptu is an improvisation, as you know) we’re not talking about being in several instances of readiness or exposing to view. No, it’s another case – like omnibus, rebus, ignoramus, and vade mecum – of a word being made into an English noun that might seem like it’s a direct borrowing of a Latin noun but is in fact grammatically different in the original source. So the plural of impromptu is impromptus, and there’s no alternative.
But because we have long learned that simple regular English plurals are to be dispreferred whenever possible, and especially that Latin words absolutely must be pluralized in an ostentatiously Latin way, darlings, lest we sound like utter ignorami… well, some of us are sometimes prone to produce spurious Latin-style plurals impromptu.
Sesquiotica 