An ingenius genious

I have seen the spelling

genious

enough times to notice it. It seems to be used either by mistake or sarcastically in response to something someone else has posted. It’s not a variant spelling; it’s plain wrong, which varius other people on the internet have pointed out. But inquiring linguistic minds want to know why. 

–ius is a vary rare English suffix. In fact, it is arguable whether it is an English suffix. Dictionary.com lists 13 words ending with –ius, of which genius, radius and trapezius are the most common. All of them come directly from Latin, and some would only be found in ancient Roman contexts, for example denarius. All of them are nouns (as far as I can tell), but –ius is not a productive noun suffix. We can’t create new English words with it, unless we are trying to evoke an ancient Roman mood.
–ious is a common English suffix. Dictionary.com lists 276 words, including various. Most of them come directly or indirectly from Latin, but there is no restriction on the contexts in which they can be used. All of them are adjectives (as far as I can tell), and –ious is a moderately productive adjective suffix. Some unknown person in the 19th century coined bodacious and Roald Dahl coined vermicious knid.

The relevant Latin adjectives had the forms -ius and -iosus, seemingly interchangeably, but the path from Latin to English is obscure because online sources don’t give examples from every step through Old French, Anglo-French and Middle English. The modern French equivalents are génie (compare Arabic jinn and English genie) and divers (compare diverse), which doesn’t help, but see furieux/furieuse. 

In You are a genius, genius is undoubtedly a noun. In That is a genius comment, it is still a noun but looks, sounds and feels more like an adjective (indeed, some dictionaries define attributive uses of nouns as adjectives). If any change of spelling ever happens, it will be that the second use becomes genious and the word becomes a genuine adjective. But not if word processor programs can help it – Pages for Mac just changed genious to genius and is red-underlining it now I’ve changed it back. Genius as a head noun is unlikely to change spelling, and all those –ious adjectives are simply never going to become –ius. 

Complicating all this is ingenious, which is undoubtedly an adjective but which is more distantly related, coming from genus and not genius (though those two words are related further back). So some geniuses are born and others are made.