Never mind is used as a conjunction to mean “let alone” and, imperatively, to suggest disregard—for example, “Never mind him” or “Never mind, I’ll do it.” The single-word or closed form nevermind is a noun meaning “concern” and is seen in the example “pay him no nevermind.”
https://www.merriam-webster.com/grammar/origin-of-never-mind-nevermind-and-nvm

