Definition of 鬼 (Oni)
鬼 (おに, oni) is a mythical Japanese creature that has two horns and scary face with tusks (see the photo). Each Oni has different a skin colour, but the red one is by far the most common (followed by blue/yellow).
Originally, 鬼 is a Chinese word that means “ghost/spirit of the dead”, and some advanced Japanese expressions use it in this sense, e.g. 鬼籍 (きせき)に入(い)る meaning “to die”, where 鬼籍 means “a fictitious book that registers information of dead people”.
Idioms/Proverbs about 鬼 (Oni)
鬼 has been a symbolic creature of power and horror in Japan, and appears in a variety of Japanese proverbs/idioms.
鬼のように (oni no youni) literally means “like 鬼” and figuratively “very/really/like crazy”. For instance, “鬼のように勉強（べんきょう）する” means “study like crazy” or “study so hard”.
Young people often abbreviate 鬼のように as just “鬼” and use it as a casual intensifier, like マジ (maji) meaning “so/really”. For instance, if someone says “鬼強い”, it can have two completely different meanings: one is “鬼(が)強い” (“One is strong”), and the other is “(something/someone is) so/really strong”. You can even say “鬼かわいい” to mean “so cute”, despite the fact that Oni is an antithetical creature to the concept of “kawaii“.
I got so/really tired.
鬼電 (おにでん) [+する]
鬼の目にも涙 (oni no me nimo namida) literally means “teardrops even from Oni’s eyes”, and figuratively “People who appear merciless (like 鬼) could cry and feel for others”.
I heard that my boss who is always very stern cried a lot at his daughter’s wedding. That’s exactly “teardrops even from Oni’s eyes”.
鬼に金棒 (oni ni kanabou) literally means “Oni with 金棒”, where 金棒 means a strong bat-shaped metal weapon, usually with lots of spikes (the weapon the 鬼s hold in the photo). Figuratively, it illustrates the situation when dominantly strong people or things attain even more power. For instance, it is “Oni with a metal bat” if a powerhouse sports club poaches a talented player from another team.
あのクラブが彼(かれ)を獲得 (かくとく) したら、鬼に金棒だ
If the club acquires him, that’s Oni with a metal bat
来年の事を言えば鬼が笑う (rainen no koto wo ieba oni ga warau) literally means “If you talk about next year, Oni will laugh” and figuratively “Since nobody knows what will happen in the future, there is no use of worrying or thinking about things in the next year”. You can also say just “鬼が笑うよ” when someone talks about their plan in the next year.
Are you planning to start studying Japanese next year? Hah, (if you talk about next year), Oni will laugh.
鬼の居ぬ間に洗濯 (oni no inumani sentaku) literally means “do laundry while oni is not here” and figuratively “do whatever you want while someone scary is away”. For instance, a henpecked husband may think “Let’s do laundry while Oni is not here” and indulges himself into his hobbies while his wife is going shopping outside. When you want to use this proverb in a sentence, you may replace 洗濯 with what you actually do (see the example below).
Since my father went golfing, let’s play TV games while Oni is absent.