[Grammar] He's genius

[Sneymarin]

Hello, I was chatting in a Discord server and someone wrote the following sentence: "he's genius"
Another corrected him and said that the correct way to say it is "He's a genius" as it's a noun, but I joined the conversation and said that informally "genius" can also be an adjective and mean "very clever or ingenious". Oh boy was that a bad move. Several people intervened and said I was wrong. When I asked for an explanation I was met with "It's just wrong. Because I have grew up speaking the language all my life I know what I’m talking about" and no matter how hard I tried to get an explanation out of them I still failed in the end. I would be grateful if someone could explain to me why saying "he is genius" is wrong. I understand that it may sound unnatural to some, but I just want to understand wheter it's a valid use of "genius" or not.

Part of the conversation for context:
J: Do my homework
B: Peter will do your homework, John. He's genius. He likes doing homework
P: Debatable. The homework part, of course.

Thank you for your time.

 

[jutfrank]

It's wrong in the sense that we don't use the word like that.

Although the adjective is commonly used as a loose synonym for 'very clever' or 'ingenious' when describing plans, creations, ideas, etc., we don't use it to describe people.

 

[Sneymarin]

But you could, technically, right?

 

[GoesStation]

But you could, technically, right?

It's not currently done. In recent slang it's become possible to describe an idea or innovation as "genius": That chocolate-garlic hummus is genius! The person who thought of it still has to be described as a genius.

 

[jutfrank]

But you could, technically, right?

No, it's not right.

(I don't know what you mean by 'technically'.)

 

[Sneymarin]

No, it's not right.

(I don't know what you mean by 'technically'.)

I mean by the definition of "genius" as an adjective. "People can be simply genius sometimes" doesn't sound strange or wrong to me. I think replacing "genius" with "very clever" or "ingenious" would make it sound worse. What do you think?

 

[GoesStation]

I mean by the definition of "genius" as an adjective. "People can be simply genius sometimes" doesn't sound strange or wrong to me. I think replacing "genius" with "very clever" or "ingenious" would make it sound worse. What do you think?

The recent use of genius as an adjective only works when you can mentally add the idea of before the noun. Is that possible when the noun refers to a person?

 

[Sneymarin]

The recent use of genius as an adjective only works when you can mentally add the idea of before the noun. Is that possible when the noun refers to a person?

I don't quite understand what you mean with "the idea of". Could you provide an example?

 

[jutfrank]

How much clearer do you want us to be? The original sentence He's genius is incorrect. The person writing it was either an English learner who made a grammar error, or a native speaker who forgot the article.

 

[GoesStation]

I don't quite understand what you mean with "the idea of". Could you provide an example?

Let's phrase it a bit differently and say "this idea, xxx, is a work of genius". Suppose you're really impressed by a new concept: chocolate-garlic hummus. You think it's brilliant! (I'm starting to worry about you, but that's a different subject.) In formal English, you might say "This idea, chocolate-garlic hummus, is a work of genius!" The current slangy way to say this is "Chocolate-garlic hummus is genius!"

This delightful snack was invented by the Cajun chef Casey Dégueulasse. You wouldn't say "This idea, Casey, is a work of genius." Casey is a person, not a concept.