Never mind tells someone to disregard a matter. It can also mean “not to mention” or “certainly not.” Never mind should be two words in almost all contexts. Nevermind (one word) is part of the colloquial expression “[pay something] no nevermind.”

Is nevermind a real word?

Never mind is most often spelled as two words—unless it is used as a noun. The closed (or hyphenated) noun form, nevermind, is dialectal and has the meaning of "interest," "concern," or "attention." It, like its conjunction cousin, is used in contexts implying indifference, disregard, or dismissal.

Is it rude to say Nevermind?

Never mind isn't rude on its own, but you do want to be sure you are using the right tone of voice when you say it. If you say “never mind” rudely, then it's obviously going to come across badly.