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It's not the case with 〜ではあるまいし

Grammar: Reason and Cause » 〜ではあるまいし

〜ではあるまいし expresses a cause or reason. It follows a noun or phrase, and is used to say “Because it is not ~” or “Because you are not ~”.

In the case of expressing a reason, a criticism, warning, advice, or an order may follow after し. However, it’s often the case that the portion after し is omitted.

When following a verb, ん or わけ may appear before で.

The colloquial counterpart is 〜じゃあるまいし.

Further Reading

Examples

It's not like I'm a grade-schooler with 〜じゃあるまいし

からかい上手の高木さん » Volume 8 » Page 122

Seeing a frozen puddle on the ground, Nishikata is excited at the thought of picking it up and holding it. Takagi asks if this is what he’s thinking, and he quickly denies it.

西片(にしかた):
「そんな… 小学生(しょうがくせい)じゃあるまいし。」
It's not like I'm some kind of...grade-schooler.”
高木(たかぎ):
「ふーん。ホントかな。」
“Hm, I wonder...”

It's not like this is science fiction with 〜じゃあるまいし

名探偵コナン » Volume 3 » Page 147

Evidence piles up for Ran that Conan, the smart young boy staying with her family, may actually be Shin’ichi, her missing childhood friend. She’s convinced that he’s Shin’ichi on the inside, but the sticking point is how he went from being a teenager to a child.

(らん):
SF(エスエフ)じゃあるまいし…」
It's not like this is sci-fi.”
「こんな不思議(ふしぎ)(こと)ってありえない…」
“Such an incredible thing is inconceivable...”

It's not like you're a foreigner with 〜じゃあるまいし

名探偵コナン » Volume 1 » Page 61

Shin’ichi’s teenage friend Ran enters Professor Agasa’s house looking for Shin’ichi, unaware that he was forced to take a poison that inadvertently turned the teenager into a young child. When Ran sees the boy and asks his name, Shin’ichi recalls Agasa’s suggesting that he not reveal who he is. Panicking, he gives the name “Conan”, explaining the oddness of it by saying his father is a fan of author Arthur Conan Doyle.

(らん):
「コナンねー…」
“Conan, huh?”
阿笠(あがさ):
「なにがコナンだ!! 外国人(がいこくじん)じゃあるまいし
“What's with 'Conan'!? It's not like you're a foreigner...”
コナン:
「しかたねーだろ? ほかに(おも)いつかなかったんだから…」
“It couldn't be helped, all right? I couldn't think up anything else...”