鬼
Oni
おに
Oni are the fearsome demons of Japanese folklore — think big, horned, and terrifyingly powerful. That image of overwhelming strength got borrowed as a slang prefix meaning 'ridiculously' or 'insanely.' Oni-tsuyoi (insanely strong), oni-kawaii (crazy cute), oni-muzui (brutally hard). It evolved from the longer phrase 'oni no you ni' (like a demon), which got shortened to just 'oni.' It even attaches to nouns: oni-den (oni-calling = blowing up someone's phone). Firmly established among young people since the 2010s.
Examples
今日のテスト鬼ムズかったんだけど
Today's test was oni hard, seriously.
Talking to friends after an exam
あのラーメン屋、鬼うまいから行ってみて
That ramen place is oni delicious — you gotta try it.
Recommending a restaurant to a friend
上司が鬼のように残業させてくる
My boss makes me work overtime like an oni.
あのゲームのラスボス鬼強い
The final boss of that game is oni strong.