Expressing that something 'is not'; 'is not the case'.
Long explanation:
The じゃあるまいし grammar point is a somewhat masculine and colloquial expression used to imply that a situation is not the case. It can be translated as 'it's not like...', 'there's no need to...', or 'it's not as if...' in English. It's generally used after a noun or a na-adjective.