The Meaning of ナイ（です）

Question

From: 声が出なくなったので、会社を辞めて二人暮らし始めました。

Context: Sora stayed over at Ema's house. Ema quit her job two weeks ago and has not cleaned up her house since then.The morning after his stay, Sora says this during breakfast:

「この家、物だらけで整理整頓されてないし、掃除もちゃんとできませんよね？というか、正直な感想を言うと、すごく汚いです。ナイです。」

At first I thougt 「ナイ」was more or less used to emphasize 「汚い」; however as Ema, comments (not in dialogue, but as narrator) with:

第三者に「ナイ」とまで断じられてしまうと、さすがにちょっとへこむ。

I kind of have the feeling that 「ナイ」has meaning on its own. From what I can guess it is meaning-wise related to 「汚い」and probably very casual.

In any case, what does 「ナイ」mean here and in general？

Answer 1

ない is a slang, and means the thing/person/place is extremely bad, to the point it's absolutely and immediately excluded from further consideration, with no chance of reconsideration.

E.g. if you say あいつはないわ, then it means your opinion of that person is so low, that you never want anything to do with them. It's used for lighthearted situations, too though - for example: 「チョコレートに醤油ってうまくね？」「ないわー」

In this case, the object is the house, so it's a very strong encouragement to do something about its state.