ではあるまいし・
You deny something that obviously isn't so, often an absurd comparison, to scold, advise or reassure: "you're not a child, so…". It is spoken, and じゃあるまいし is the usual shape.
You're not a child. Do that much yourself.
We're not professional climbers. There's no need to push for the summit.
After a noun
Noun + じゃあるまいし ／ ではあるまいし
ではあるまいし is the careful shape, and the written one. でもあるまいし is also said: 子どもでもあるまいし.
After a verb
Plain form + わけじゃあるまいし ／ んじゃあるまいし ／ のではあるまいし
死ぬわけじゃあるまいし, 外国に行くんじゃあるまいし. A verb never joins it directly.
- Before it
- Something you both know isn't so, often far-fetched: 子ども, 神様, 魔法使い, 死ぬわけ.
- After it
- Your verdict: an order, advice, reassurance, or a retort.
- Tone
- Blunt: you should know better. Fine with family, friends and juniors, and about yourself; rude to someone above you.
- Where you'll meet it
- Conversation, dramas, manga, the dialogue in novels.
In use
You're not new here any more. Make a call like this on your own.
I'm not God. There's no way I could know that far ahead.
About yourself it is a retort: don't ask the impossible of me.
It's not as if anyone is going to die. You don't need to worry so much.
People often stop at じゃあるまいし and leave the rest unsaid.
Don't cry over a thing like that. You're not a child.
It leads to "so", not "yet"
The denial is your grounds: you're not a child, so do it yourself. A plain statement of what is so in spite of it takes のに.
Not said: プロじゃ
(He isn't a professional, yet the cakes my brother makes are delicious.)
プロじゃないのに.
Close neighbours
〜じゃないんだから makes the same push in plain words, and is the safer one to say yourself.
You're not a child any more, so think it through for yourself.
わけではないN3
"It's not that…": it gently takes back a conclusion, and nothing has to follow. No scolding in it.
It's not that I particularly dislike it, but I don't eat it much.
ではあるまいかN2
One kana away, and nothing alike: it ends a written sentence and means "might it not be…?".
Might this not be a coincidence?
Check yourself
1Which sentence sounds natural?
Choose an answer to see why.
2Which one fits?
明日世界が
Choose an answer to see why.
More detail
Word for word it is ではないだろうし: あるまい is a written "surely isn't". It is the まい you are most likely to hear in conversation.
More sentences
You're not a child. You should take responsibility for your own mistakes.
It's not as if every second counts. Please check the contents first, and then send it.
It's not as if someone is watching you. You don't need to worry so much about the people around you.
You're no wizard. There's no way you could know how someone feels when nothing has been said.
This isn't an exam. Let's talk over anything we don't understand as we go.