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I learnt that the negative form of い-adjective is: remove the trailing い, then
add くない. Example: 赤い→赤くない

However, I've been watching Japanese livestream and following her twitter.
Sometimes, she omit the く in negative form of い-adjective. Example: [from her
Twitter post](https://twitter.com/niboko_niconico/status/857865239144013824)

> キャベツ **美味しない** 。千切りの方が好き

In her livestream, she also sometimes said やばない instead of やばくない, and 酷ない
instead of 酷くない.

That makes me wonder if this is somewhat common in Japan (perhaps dialect?),
or just her habit. If it matters, she's from Kumamoto.
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colloquial-language dialects negation i-adjectives
Omission of く in negative form of い-adjective
"やばない？" instead of "やばくない？" is kind of slang (of slang) which some young
people use. But I don't often see this omitting "く" with other adjectives. The
example "キャベツ美味しない。" doesn't make sense to me, so I think it's her mistake or
just her habit.
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2017-04-28T11:12:21.147
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> 赤｛あか｝くない ー 赤｛あか｝ない ー not red  
>  美味｛おい｝しくない ー 美味｛おい｝しない ー not delicious

I often heard like the sound in Kansai.

They don't pronounce them as "aka-nai" and "oishi-nai", but pronounce them
like "aka-a-nai" and "oishi-i-nai" or "aka:nai" and "oishi:nai".

where ":" means to sound the vowel long

As the samples show, they pronounce them by omitting "く" and adding the same
vowel or sounding the vowel long just before the く in the formal forms.
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There are at least three types of omission of く, which should be
distinguished.

The "traditional western" euphoric change is called ウ音便 and is described in
[this question](https://japanese.stackexchange.com/q/24217/5010), [this
one](https://japanese.stackexchange.com/q/42890/5010) and a chart in [this
page](http://www.akenotsuki.com/kyookotoba/bumpoo/keiyooshi.html). `ku`
becomes `(y)u`, etc. This sounds old-fashioned and elegant. While this is
commonly heard in samurai dramas, only a few courteous elder people use this
today.

>   * おいしくない → おいしゅうない
>   * くるしくない → くるしゅうない
>

In the (modern) Kansai dialect, instead of saying `ku`, the vowel right before
`k` will be elongated:

>   * おいしくない → おいしいない
>   * くるしくない → くるしいない
>   * あつくない → あつうない
>   * からくない → からあない
>   * くろくない → くろうない (kuronai)
>

The "recent slangy" version is different from these. This pattern is typically
found on Twitter and such and look like this:

>   * おいしくない？ → おいしない？
>   * やばくない？ → やばない？
>   * ひどくない？ → ひどない？
>

It's similar to the Kansai dialect, but no elongated vowel is employed. And I
doubt this pattern is dialectal; I often see young people living in Kanto say
things like these. I believe this is a very new phenomenon (it suddenly
emerged in the last 5 years or so) among young people, and AFAIK this is
mainly used as a question sentence: "Isn't it ～～?" I am in my thirties and I'm
perhaps already too old to use this naturally :D

As for 美味しない in the tweet in question, the user is not a Kansai dialect
speaker. So it's not archaic elegant 美味しゅうない, nor typical Kansai 美味しいない, but
recent slangy 美味しない. I think most young people who say these are doing so more
or less playfully, knowing it's not standard. For now I don't think this will
be part of the formal Japanese grammar in the future.
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