Flat Adverbs

In grammar, a flat adverb is an adverb that looks like an adjective, or, more specifically: it refers to taking a word that has both an adjective form and an adverbial form, and using its adjective form (instead of the adverbial one) to express the adverbial meaning. In English, adjectives become adverbs through the "~ly" suffix, so saying "it went really bad" instead of "it went really badly" would be using "bad" as a flat adverb.^{(Eversoll, 2014:1)} In Japanese, a similar phenomenon occurs with a few intensifying adjectives like sugoi すごい, whose adverbial form (ren'youkei 連用形) would be sugoku すごく, that are nevertheless used in the adjective form to express their adverbial function sometimes.

Anime character wearing shirt saying Sugoi Dekai

Character: Uzaki Hana 宇崎花
Anime: Uzaki-chan wa Asobitai! 宇崎ちゃんは遊びたい！ (Episode 1)

Context: an anime girl with a lip fang and a shirt spelling in romaji:
{sugoi} dekai
すごいでかい
{Incredibly} big.
- {sugoku} dekai
  すごくでかい
  (same meaning.)

Grammar

In English, there are various sorts of flat adverbs, and some tend to occur specially together with an intensifier, e.g.:

He sings well. (not a flat adverb.)
He sings good. (flat adverb, this sounds odd.)
He sings pretty good. (intensified flat adverb, this is more normal.)
He sleeps quiet. (vs. sleeps quietly.)
He thinks quick. (vs. thinks quickly.)

The term flat adverb generally refers to sentences like above, but I think there are other two similar types of sentences worth taking note of.

First, when the word express the final state after an action, it's always in adjective form in English:

It burned hot. (not hotly.).
The ship sank deep. (not deeply.)

Second, when they're the complement of a copula in English, they'll also be in adjective form:

He is/stays/continues/remains/becomes quiet/quick. (not quietly or quickly,)

In Japanese, the copula and the final state uses are always in adverbial form:

atsuku moeru
熱く燃える
To burn hot.
fukaku shizumu
深く沈む
To sink deep.
hayaku naru
速くなる
To become quick.

Meanwhile, while what's normally called a "flat adverb" in English also exists analogously in the Japanese language, the Japanese flat adverb is EXTREMELY limited in usage. You'll basically only see sugoi used like this.

sugoi tanoshii!
すごい楽しい！
It's very fun!
sugoi oishii
すごい美味しい
It's very delicious!
sugoi sugee!
すごいすげぇ！
It's incredibly incredible!

First, only i-adjectives can be used as flat adverbs—na-adjectives and no-adjectives can not, despite having both adjective and adverbial forms.

Second, only a handful of i-adjectives with intensifying meaning can be used as flat adverbs. These include:^{(増井典夫, 1987:78, as cited in keio.ac.jp/~rhotta:real bad と「すごいヤバい」)}

erai
えらい
Important. Distinguished.
(generally used in the sense of something became a "big trouble," such that authorities had to be called.)
osoroshii
おそろしい
Terrifying. Terrible. Terribly.
sugoi
すごい
Incredible. Awesome.
mono-sugoi
ものすごい
(same meaning.)

And also:

yabai
やばい
Dangerous. Extremely.

Japanese natives may think that the flat adverb usage is grammatically wrong, and, also, that this usage is something that appeared recently, i.e. it's the youngsters speaking the language all wrong. However, this flat adverb usage isn't new, and has existed in Japanese for centuries.^{(増井典夫, 1987:78, as cited in keio.ac.jp/~rhotta:real bad と「すごいヤバい」)}

For reference, a couple of examples:^{(The Balanced Corpus of Contemporary Written Japanese)}

"anta no okaa-san, ikutsu ya?" "kotoshi, yon-juu-san desu" "e'? {erai} hayai-n-tochuu?"
「あんたのお母さん、いくつや？」　「今年、四十三です」　「エッ？えらい早いんとちゃう？」
"How old is your mother?" "forty three this year." "Eh? {Too} early, [don't you think]?"
- Although I don't know the context, it probably means "too early" in the sense of "she's too young."
- tochuu is dialectal for janai じゃない, "isn't it," "don't you think."
- Source: 典奴どすえ, 1987.
saki-hodo no jichishou no go-touben dewa, {chihou-jichitai ni {futan wo suru} kinou wo ataeru} toiu fuu na, {{erai} muzukashii} o-kotoba ga gozaimashita.
先ほどの自治省の御答弁では、地方自治体に負担をする機能を与えるというふうな、えらいむずかしいお言葉がございました。
Just now in the ministry of home affair's response, {to give local municipalities the ability {to bear [a cost or responsibility]}}, such {{extremely} difficult} words existed.
- In the sense that such words were in the response, it contained such words, such words were said
- Source: 国会会議録, 1983.