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

Grammar Explanation: It's not the case with 〜ではあるまいし

Shin’ichi’s teenage friend Ran enters Professor Agasa’s house looking for him, 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 suggestion to 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.

欄(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 likeyou're a foreigner...”
コナン:: 「しかたねーだろ？　ほかに思(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...”

Key Points

外国人(がいこくじん)じゃあるまいし = it’s not like you’re a foreigner
- Agasa is criticizing the obviously foreign-sounding name Conan.
- The phrase implies “you’re Japanese, so why would you use a name like that?”
じゃあるまいし rejects a premise as inapplicable
- Agasa brings up one category, being a foreigner, and dismisses it as not the case here.
- That makes the chosen name sound unreasonable or absurd.
なにがコナンだ conveys exasperation
- This opening is not a literal information question.
- It is a scolding “what’s this ‘Conan’ supposed to be?”