This pattern is used to state that something is not the case, and therefore the other person's behavior or expectation is unreasonable. It's similar to saying 'it's not like...' in English. For example, if someone is treating you as if you were a child, you might say '子どもではあるまいし' meaning 'It's not like I'm a child (so stop treating me like one).' It often carries a tone of mild frustration or rebuke.

For verbs, you attach it to the dictionary form: '行くではあるまいし' would mean 'It's not like (I) will go.' But note that in practice, this pattern is more common with nouns and な-adjectives. For nouns and な-adjectives, you simply place the word before ではあるまいし without any copula: '学生ではあるまいし' (It's not like you're a student). For い-adjectives, you would need to use the plain form but note that the pattern is used with nouns and な-adj most often.

This is a rather formal and literary pattern. In everyday conversation, you'd likely hear 'じゃあるまいし' (the contracted form of では) or more casual alternatives like 'わけじゃないんだから'. So it's good to understand it for reading and N1 exam, but use it sparingly in speech.