ではあるまいし・
This construction rejects an excuse, comparison, or supposed situation and then states what should reasonably follow: “It is not as if X, so...” It often introduces criticism, advice, a command, or a firm judgment.
How the reasoning works
The first part names an excuse or comparison and denies that it applies. The second part then gives the behavior or judgment that becomes reasonable once that excuse has been removed. A useful mental model is: “You are not X, so do not act as though you were.”
A noun-based comparison
子供ではあるまいし、自分の失敗には責任を持つべきだ。
You are not a child, so you should take responsibility for your own mistakes.
AnalyzeThe speaker rejects “being a child” as an available excuse and follows with a firm expectation.
The conversational contraction
もう新人じゃあるまいし、これくらい一人で判断してよ。
You are not a new employee anymore, so make this much of a decision on your own.
Analyzeじゃあるまいし fits the direct conversational tone, but the sentence is clearly scolding.
Using a verb clause
A verb clause does not normally attach directly. Turn the situation into a noun-like expression with の, conversational ん, or わけ first. Choose the one that makes the supposed situation natural in context.
誰かに見張られているのではあるまいし、そんなに周りを気にしなくてもいい。
It is not as if someone is watching you, so you do not need to worry so much about everyone around you.
AnalyzeDo not confuse the ending
ではあるまいし rejects something and uses that rejection as a reason for the following judgment. By contrast, sentence-final ではあるまいか tentatively proposes an inference. Also compare のだから, which treats its preceding reason as established rather than denying that an excuse or comparison applies.
More examples
私たちはプロの登山家ではあるまいし、無理に頂上を目指す必要はない。
We are not professional mountaineers, so there is no need to force ourselves to reach the summit.
Analyzeあなたは彼の家族じゃあるまいし、そこまで世話を焼く必要はないよ。
You are not his family, so you do not need to look after him to that extent.
Analyze一分一秒を争うわけではあるまいし、まず内容を確認してから送ってください。
It is not an emergency where every second counts, so please check the contents before sending it.
Analyze誰も君を責めているんじゃあるまいし、そんなに身構えなくてもいい。
It is not as if anyone is blaming you, so you do not need to be so defensive.
Analyze君は魔法使いじゃあるまいし、何も言わずに相手の気持ちが分かるはずがない。
You are not a magician, so you cannot be expected to understand how someone feels without being told anything.
Analyze試験ではあるまいし、分からないところは相談しながら進めよう。
It is not an exam, so let us discuss anything we do not understand as we proceed.
Analyze