なさい

Etymology
From, derived from.

Suffix

 * 1)  combines with the verb stem to create an imperative as a kind of strong request

Usage notes
This is a suffix, attaching to the of verbs.

This is stronger than a simple request, and is considered somewhat informal. This may be used as a way of issuing a command, such as a teacher prompting a student to pay attention, or a parent scolding a child.

Unlike, -nasai can only be used to make positive commands, never negative commands.