ではあるまいし
dewa arumaishi
it's not like; you're not... (so don't)
[It's not like / You're not (so don't)]
Connection: Noun + ではあるまいし / Verb の + ではあるまいし
Usage: Used to dismiss someone's behavior or a situation by pointing out that it is inappropriate given the reality. Implies "you're not X, so you shouldn't act that way" or "it's not X, so there's no reason to do that."
Examples:
子供ではあるまいし、そんなことで泣くな。(You're not a child, so don't cry over something like that.)
王様ではあるまいし、そんなわがままは通らない。(You're not a king — you can't get away with being that selfish.)
初めてではあるまいし、もう少し落ち着いてやりなさい。(It's not your first time, so calm down and do it properly.)