ではあるまいし expresses “it is not as if ...” or “you are not ...,” rejecting an assumed excuse or inappropriate behavior. It commonly presents a person or situation as obviously not having a certain status, condition, or circumstance, followed by advice, criticism, a command, or a conclusion that follows from that rejection. The expression is rhetorically forceful and somewhat formal or old-fashioned, and it is especially common in written language and admonishing statements.