The standard spelling of this phrase is as two words: “never mind.” The popularity of the alternative one-word form “nevermind” was certainly enhanced by its use in 1991 as the title of a bestselling Nirvana album. “Nevermind” can look immature or slangy to some readers. You can still be cool by imitating the vocabulary choice in the title of another famous album: Never Mind the Bollocks: Here’s the Sex Pistols.

In expressions like “pay him no nevermind” where the word means “attention” it’s always one word, but those expressions are both slangy and old-fashioned.