GRAMMAR DETAIL

ではあるまいし

you are not · it is not as if

Criticizes behavior as inappropriate because the person is not in the role or situation that would excuse it.

Examples

子どもではあるまいし、自分の責任は自分で負いなさい。

You are not a child, so take responsibility for your own actions.

学生ではあるまいし、自分で調べなさい。

You are not a student, so look it up yourself.

Structure: Attach ではあるまいし to a noun naming a role or situation to criticize behavior as inappropriate because that role or situation does not apply.
Basic usage: 子どもではあるまいし、自分の責任は自分で負いなさい。 rebukes someone for avoiding responsibility like a child and tells them to take responsibility themselves.

Grounds for criticism rather than uncertainty
- Here 子どもではあるまいし uses the fact that the person is not a child as grounds for rebuke rather than expressing uncertainty about their age. Read it with the command or advice that follows.
Tone toward the listener
- It can sound more reproachful than 子どもではないので. Because it may sound condescending, it is not a suitable default for a polite request.