Explanation
Means 'it’s not as if...', 'it’s not that...', more formal than じゃあるまいし.
Formation
Noun + ではあるまいし
Detailed explanation
「Noun + ではあるまいし」indicates that the noun is not the situation or reason being implied. It dismisses a possibility or justification, e.g. 'It’s not as if X, so don’t worry/do that/etc.' This is the more formal counterpart to じゃあるまいし.
Examples
Kanojo wa puro no kashu de wa aru mai shi, kanpeki o motomeru no wa koku da.
She’s not a professional singer, so it’s rather harsh to demand perfection.
Kare wa kodomo de wa aru mai shi, sonnani chokoreeto bakari taberu no wa douka to omou.
He’s not a child, so I’m not sure about him eating that much chocolate.
Kare wa isha de wa aru mai shi, anata no kenkou joutai o sensaku suru hitsuyou wa nai.
He’s not a doctor, so he has no need to pry into your health.
Kono yubiwa wa kouka na mono de wa aru mai shi, otoshite mo taishita koto wa nai.
It’s not an expensive ring, so dropping it isn’t a big deal.
Practice
Grammar content sourced from hanabira.org.