〜ではあるまいし

Meaning

Means "since it is not ~," citing an impossible matter and using it as grounds for the following judgment. A somewhat colloquial expression carrying criticism or admonition.

Examples

子どもではあるまいし、それくらい自分で考えなさい。 You are not a child, so think about it yourself.
神様ではあるまいし、未来のことなど分かるはずがない。 I am not God, so how could I possibly know the future?
新人ではあるまいし、こんな初歩的なミスをするとは。 You are not a newcomer, so it is hard to believe you would make such a basic mistake.