Ah, the constant lexical capriciousness of the English language. It’s like a combination of Christmas and Hallowe’en: when you unbox a word, you don’t know if it will be trick or treat. It’s like trying to learn to eat with a full formal flatware setting: the multifarious forks, knives, and spoons, there for you to choose from wrongly and make yourself look like a bumpkin. Our derived word forms in particular display a pointed obnoxion.
What do you mean obnoxion isn’t a word? Religious – religion; contagious – contagion… What is it, obnoxity? You know, loquacious – loquacity; mendacious – medacity… Well, then, how about obnoxiety? Like anxious – anxiety? Let’s see… calamitous – calamity could give us obnoxy… It’s not obnoxicity, anyway, right, because that really is just an –ity with an –ic before it… Maybe obnoxiosity on the model of monstrous – monstrosity? No?
Well, we’re getting through all the Latin endings here. I mean, the word obnoxious comes from Latin obnoxius, which is from ob ‘towards, in the way of’ and noxa ‘hurt, injury’; it meant ‘exposed to harm, answerable, subject to punishment’. So clearly we should use a Latin-derived suffix to make the noun. It would be silly to stick on an old Germanic suffix, right? Something like –ness?
Oh, come on, you have got to be kidding. Obnoxiousness? That’s the correct form? Well, it sure is an obnoxiously long word, so maybe it’s fitting. Anyway, there are other –ous words that get a –ness as well; the Oxford English Dictionary lists hundreds of them, from abstemiousness through anxiousness and atrociousness and barbarousness and boisterousness and capaciousness and consciousness and contagiousness and… um, yes, we do have anxiety and atrocity and barbarity and capacity and contagion…
And, according to Oxford, we also have obnoxity and obnoxiety. Could it be that obnoxiousness is a measurable quality and one or both of the others is an instance of someone or something being obnoxious? Well, yes, an obnoxity is “An obnoxious, objectionable, or offensive person or thing; an object of aversion.” As to obnoxiety, it’s listed as “Originally: †the state of being liable to something; liability (obs.). Now: offensiveness, objectionableness, odiousness.”
Remember: the original definition of obnoxious in English as of the 1500s, and the usual sense before the 1800s, was (per Oxford) “Liable, subject, exposed, or open to a thing (esp. something actually or possibly harmful).” You would be obnoxious to punishment or obnoxious to accidents or or or… But the ‘hurtful, injurious’ sense (proper to Latin obnoxiosus) appeared in the 1600s and prevailed by the 1800s.
Mind you, these days obnoxious means not so much ‘hurtful’ as (to quote Merriam-Webster) “unpleasant in a way that makes people feel offended, annoyed, or disgusted.” We see it in phrases like obnoxious little brat, loud and obnoxious, rude and obnoxious, and obnoxious behaviour. It’s a very hard-to-ignore quality.
Which leaves us with the obnoxion of English derivational morphology. There are plenty of people who would like obnoxion to be a word in regular use; just Google it and you’ll see. But there are also people who are rather obnoxious about its being “not a word.” (“You dreadful little man, you are using the wrong fork with your squab.”) You get generally similar results for obnoxity, obnoxiety, obnoxicity, obnoxiosity (and obnoxiousity), even obnoxy.
I think it just leaves plenty of opening for being obnoxious. Which, in the bratty sense, can even include a little puckishness and rambunction. Rambunctiousness. Rambunxity?