Nyssa is a rare female name. Many variations of the name exist, including Nisa, Niisa, Anyssa, and Nys.
Nowadays. Nyssa has become much more. Nyssa is now used to describe (often in all capitals "NYSSA") any sort of person that you feel needs to be knocked down a rung or two, jokingly or not.
Most commonly used jokingly between friends, such as "YOU'RE SUCH A NYSSA, LOL" on chat sites. It has become a recent small internet "meme" and even has its own facebook group.