What it means
なさい attaches to the masu-stem of a verb to give a command or instruction. It is commonly used by parents, teachers, or other people with authority when telling someone what to do. It is often less harsh than the plain imperative form, but it still sounds commanding rather than politely requesting.
Usage note
なさい is not a general polite equivalent of a command. Avoid using it with customers, supervisors, or unfamiliar adults; use a request such as ～てください instead when a polite tone is needed.
Example sentences
母は子どもたちに、寝る前に歯を磨きなさいと言いました。
Mother told the children to brush their teeth before going to bed.
先生は学生に、教科書を開きなさいと言いました。
The teacher told the students to open their textbooks.
父は私に、宿題を先