Use of なさい in ごめんなさい

Question

Why is なさい used in ごめんなさい when it is usually used as a command form?

Answer 1

免 is a somewhat old word meaning 'forgiveness'. There is a polite command form that goes お・ご～なさい, such as in おいでなさい or ご覧なさい. If you apply this to 免, you get ご免なさい, literally 'please forgive me'. This command form is also seen in おやすみなさい and おかえりなさい.

Answer 2

Nothing at all's answer is fine the basics, but it could be misleading depending on how much Japanese you are planning to learn.

ご/おV-stemなさい is a polite command conjugation of the verb なさる.

なさる is the keigo equivalent of する (meaning it is respectful). Conversely, いたす is the self-humbling form.

So by using なさる you express respect for the person you are talking to and by using ご/おV-stemください you express it politely.

免ずる means to forgive and this is then conjugated to mean "please forgive me" by using a polite and respectful command conjugation.