"Nevermind" vs "never mind"

Nevermind is a word, but it is very outdated. Don't believe me? Type it in word and you'll see the red squigglies appear underneath it (coincidentally, it does the same with the word "squigglies").

 In common (everyday) speech you are using the two-word version, "never mind," which means to disregard or ignore. Don't use the outdated "nevermind" because, I promise you, you will use it wrong. In its correct usage, it doesn't sound like correct English.

Never mind what Tom says, he is crazy! 

Never mind the manager, he is upset with everybody every day. 

 For what it is worth, "nvm" is NOT a word.