Originally a variant of "yes", used to express joy at something unbearably sexy. Came from people's inability to type when they were so erotically excited.
Bob: "Hey John, you wanna come rob a bank?"
John: "No."
Bob: "Hey John, you wanna come to church?"
John: "No."
Bob: "Hey John, you wanna come get some drinks?"
John: "Nyes."